MOSTLY MORGENTHAUS A Family History By Henry Morgenthau III Ticknor & Fields. 501 pp. $27.50

ON THE SURFACE, at least, Mostly Morgenthaus is a fondly detailed, often humorous, intimate family memoir, covering the Horatio Alger-like rise of an immigrant family to affluence and influence in the New World. Born in poverty in 19th-century Bavaria, Lazarus Morgenthau started out as an itinerant cantor; succeeded briefly, then failed, in the cigar business; and yet was nonetheless able to bring his family to the Land of Golden Opportunity.

Once in New York, his son Henry Sr. put himself through Columbia Law School, made a tidy fortune in real estate, and was appointed American ambassador to Turkey by Woodrow Wilson. Lazarus's grandson, Henry Jr., was a gentleman farmer who, through his wife's friendship with Eleanor Roosevelt, was named FDR's secretary of the treasury and remained a key member of the cabinet until FDR's death. A great-grandson, Robert Morgenthau, was appointed the U.S. attorney for New York's prestigious Southern District by President Kennedy. And a great-granddaughter, Joan Morgenthau, was given her coming-out party at the White House, the first non-member of a presidential family to be accorded such an honor. Talk about rags to riches!

Along the way, the family strove for probity, propriety and social conformity. A great family "scandal" in the 1920s was the shocking news that cousin Morris Fatman and his wife slept in a double bed. Such sleeping arrangements were simply not done in the prim little world of the German-Jewish elite. "It was the talk of the town," a relative told the author, after asking her children whether she should reveal this "peculiar" circumstance. (Two of the children said yes, one said no.)

Of course such American success stories have become more than twice-told tales. But the Morgenthau story, as here related by another of Lazarus's great-grandsons, has a darker, more troubling subtext. As New York's German Jews emerged from the coccoon of poverty, they struggled to camouflage any traces of ethnicity that might distinguish them from the predominantly Christian community, and cause them to appear "different." They celebrated Christmas with Christmas trees and cards and caroling. They celebrated Easter with bunnies and baskets and hunts for colored eggs. The fact that cousin Irving Lehman displayed mezuzahs on the doorposts of his house, and gave Passover seders, was painful to explain. But since he was chief judge of the New York State Court of Appeals, he could not be argued with.

At the same time, the "Uptown" German Jews cautiously avoided social situations or institutions where they felt they would not be welcomed and, instead, created exclusive German-Jewish institutions of their own: Temple Emanu-El and Mount Sinai Hospital on Fifth Avenue, the Harmonie Club for men, the Century Country Club in Westchester County. If the WASPs claimed Newport as their private preserve, the German Jews would build their own vast "cottages" in Elberon, on the Jersey shore. If Saratoga Springs had been declared WASP country, the German-Jewish families would move further north, and erect elaborate summer camps in the Adirondacks. "Never push yourself forward," German-Jewish parents told their children. Don't be too visible. Eat your bacon and your ham. Enjoy your shellfish. Don't make waves.

It was called assimilation. But the Morgenthau family seemed to carry this notion to an extreme. Morgenthaus never set foot inside Temple Emanu-El, not even on Yom Kippur. They refused to be doctored by physicians on Mount Sinai's staff. The Morgenthaus declined memberships in the Harmonie and Century clubs. They avoided the Adirondacks and the Jersey shore. They chose instead to vacation at an anonymous farm in Dutchess County, or at the Quaker-owned Weekapaug Inn in Rhode Island. "I don't remember ever being told in so many words that I ws Jewish," the author writes. "It just seeped into my consciousness." But he does remember his grandfather giving his father this advice: "Don't have anything to do with the Jews. They'll stab you in the back." And, as a child of 5, the author recalls being asked by a playmate what his family's religion was. Since he didn't know, he asked his mother. She looked pained. "If anyone ever asks you that again, just tell them you're an American," she said at last. And so he grew up convinced that his Jewishness was something shameful, "a kind of birth defect that could not be eradicated," but one that, with luck, might go unnoticed.

As a result of all this careful ethnic laundering, Henry Morgenthau III entered young manhood unready for some unpleasant surprises. When he arrived at Princeton in the fall of 1935, he assumed that, as the son of a highly placed cabinet officer, he would be invited to join one of Princeton's select eating clubs. On the night of the "bicker," when upperclassmen paid their calls on sophomores to interview them for club memberships, young Morgenthau waited in his dormitory room -- in his button-down shirt, knit tie, blazer, doeskin slacks, and fashionably scuffed white buckskin shoes -- ready for his callers. Up and down the corridor were the sounds of upperclassmen's footsteps. Young Morgenthau waited expectantly for the knock on his door that would mean an invitation. No knock came. Morgenthau relates this humiliating experience in a chapter titled "Princeton, a Painful Awakening."

And yet, at this point in the narrative, the reader may express a certain amount of disbelief. Why was this awakening so painful, and why had it taken so long to come about? These, after all, were the 1930s. Nazism was on the rise, and Adolf Hitler had been named Time's Man of the Year. Social antisemitism was rife -- even chic -- throughout the civilized world. Jews were routinely excluded from college fraternities, country clubs -- even entire suburbs -- across the country. Elite New England boarding schools (Morgenthau went to Deerfield Academy) had established quota systems for Jewish students. Several years earlier, Morgenthau's first cousin, Barbara Tuchman, had been told as a freshman at Swarthmore that she could not be taken into a sorority because she was Jewish. Hadn't young Morgenthau, or his parents, investigated Princeton's eating club policy before selecting that most snobbish of universities? Apparently not, unless -- as this reader suspects -- the Morgenthaus were afflicted with such extreme hubris that they believed that restrictions which might be applied to ordinary Jews would never be applied to them. They weren't Jews. They were Morgenthaus.

Henry Morgenthau III does not really deal with these questions in the sections of the book detailing his struggles to accept his Jewishness -- which is perhaps why Mostly Morgenthau frequently seems a bit long on self-pity, and a bit short on self-analysis.

Barbara Tuchman had handled the Swarthmore situation more forthrightly. (But, again, hadn't anyone told her that Swarthmore sororities didn't take in Jewish girls?) She simply transferred to Radcliffe, where sororities did not exist -- to the astonishment of Swarthmore's president, who insisted that social bigotry could not exist on his lovely, elm-shaded campus.

Who was the more courageous of the two Morgenthau cousins? Tuchman walked away from the problem. Henry Morgenthau III tried to wrestle with it. He joined a small group of Princetonians who met on Friday evenings for Sabbath services, conducted by a rabbi. But since he didn't understand the service, it meant nothing to him. He also discovered that many of these Friday worshippers were not Jews at all. They were canny Christians who had figured out that by getting their religious obligations out of the way early -- Sunday chapel was compulsory -- they could get to spend longer weekends off-campus.

As the 1930s progressed, and the shadows across Europe lengthened, the Morgenthaus, with their close ties to the Roosevelt White House, became aware of the plight that was facing Europe's Jews as Hitler moved inexorably toward his Final Solution. As has been written before, FDR and other world leaders knew exactly what the Third Reich was up to, but withheld the truth from the public on the grounds that fighting a "Jewish war" in Europe would neither be popular with most Americans nor politically helpful to the president himself. As a result, such relief efforts as there were from this side of the Atlantic were pathetically spotty. From his unique vantage point, Henry Morgenthau III watched this happening with the same bewildering mixed feelings as did others in the American Jewish Establishment. ALAS, looking back, and from the perspective of those sorry and unforgiven years, one must conclude that FDR was correct in his assessment of the American temperament at the time. And -- alas again! -- America's wealthiest and most influential Jews, like the author's father, who had found more success and freedom in the United States than at any point in their history, agreed to go along with America's president.

It was with his 1962 marriage to a young woman from a strictly Orthodox Jewish family that Henry Morgenthau embarked upon what he calls his "Jewish rebirth," a slow and often tortured journey into alien terrain, in hopes of discovering who he really was and where and what he really came from. For a Jew to be reborn in a family like the Morgenthaus clearly required a high-forceps delivery, and his odyssey is clearly not yet ended. Writing this book, he tells us in an introduction, has been part of this arduous, but nonetheless joyful, process. It involved more than just re-substituting Chanukah for Christmas, and Passover for Easter. Among other things, Morgenthau broke with his family's "tradition" (borrowed from the WASPs, of course) of enumerating eldest sons of a dynasty as they appeared (Henry Sr., Henry Jr., Henry III, etc.) Instead of Henry IV, Henry Morgenthau III has named his son Henry ben Henry.

Like cousin Irving Lehman's mezuzahs, his Morgenthau kin don't know quite what to make of all this. But may Henry ben Henry's tribe increase. Stephen Birmingham is the author of "Our Crowd: the Great Jewish Families of New York" and, most recently, the novel "The Rothman Scandal."