Second Marriage. by Frederic Barthelme. Penguin. $5.95. Barthelme, brother of the more famous Donald, has written a slight but funny and perceptive novel about people living in a world rarely glimpsed in serious American fiction: the world of singles apartment complexes and convenience stores and discount marts and fast foods. His attitude is not condescending but sympathetic and understanding; though the people who inhabit this world may at first seem rootless and aimless, Barthelme locates the ways they give dignity and meaning to their lives.

Airships, by Barry Hannah. (Vintage, $5.95). Like one of his characters in the story "Our Secret Home," Barry Hannah's writing could be said to have "an unsettling charm." This particular collection dates from the '70s and already has the status of a contemporary classic. As well as boasting some of the best titles since Flannery O'Connor's stories, these tales, set in the South, are wild, funny, original, bawdy and violent, barely contained by Hannah's natural flair for rhythm and timing. As a Hannah character might say, this boy can blow his horn. NONFICTION

Dear America: Letters Home From Vietnam, edited by Bernard Edelman, foreword by William Broyles Jr. (Pocket, $6.95). The letters in this heartbreaking book about a heartbreaking war are from ordinary soldiers to their parents, their wives and lovers, their friends, to the mothers of fallen comrades. They are written from places like Khe Sanh, Chu Lai, and Saigon by men and, yes, boys. Many of their names are now carved on the marble of the Vietnam Memorial. Together they offer a picture of both confusion and bravery -- confused feelings about what the war is all about, bravery in the face of certain danger which can erupt at anytime. Anyone who doesn't think war is hell should read them.

The American Conservation Movement: John Muir and His Legacy (University of Wisconsin Press, $14.95). This excellent history is an antidote to those enthusiasts who suppose that the American conservation movement was born full-grown on Earth Day, 1970. It began in the latter half of the 19th century, with giants like Muir, who called the sheep grazing in Yosemite "hoofed locusts," and a gaggle of bird-lovers who fought to halt the slaughter of plumed specimens for millinery purposes. One of the least-known and most impressive personages in the book is Rosalie Edge, the fiery Audubon activist who transformed the Society from a coterie of passive gentlefolk into a powerful force for wildlife preservation.

To Get Rich Is Glorious: China in the '80s, by Orville Schell (Mentor, $3.95). Orville Schell looks at the emerging capitalism evident in China today, from the street markets, where hawkers selling everything from sewing machine needles to gilded Buddhas ply their wares, to independent construction companies in the countryside. Schell maintains that Chinese capitalists will become increasingly like religious converts -- true believers with a passion for a new way of doing things. Where it will all eventually lead them he does not hazard to guess.

Running Without Fear, by Dr. Kenneth H. Cooper (Bantam, $3.95). When James Fixx, the high priest of running, died while jogging, a shock hit the U.S. running community. Author Cooper believes that Fixx died because he started running too late, at the age of 36, after years of sedentary living, smoking, and improper diet had given him incipient arteriosclerosis. A thorough treadmill test, Cooper theorizes, would have uncovered Fixx's problem. Putting special emphasis on a proper cool-down period after aerobic exercise, the author lays out a plan for minimizing the risks of dying while trying to keep fit. MYSTERIES

The Wrong Case, by James Crumley (Penguin, $5.95). Divorce lawyer "Milo" Milodragovitch stands in the window of his Meriwether, Montana, office and pours himself another drink: "Like most men who drink too much, I had spent most of my life considering my dismal future, and it had stopped amusing me." Soon a client appears, an attractive young woman, and Milo is off on a missing-person case, careening down Big Sky interstates. Crumley, author of The Last Good Kiss and Dancing Bear, and firmly in the heroic, hardboiled private-eye tradition of Dashiell Hammett, Raymond Chandler and Ross Macdonald, is as good as any.

The Ransom Game, by Howard Engel (Penguin, $3.50). So who said you couldn't have a Jewish hard- boiled detective? After all, the lingo of Philip Marlowe sounds an awful lot like Yiddish black humor. In this novel, Benny Cooperman sits in his Toronto office and wishes he were in Florida with his brother the doctor and his cousin the lawyer. But in time-honored fashion a beautiful dame soon walks through the door with a sob story about her gangster boyfriend and Benny is off on a new case, this time in search of Johnny Rosa and a half-million in ransom money.

The Case of the Baker Street Irregulars, by Anthony Boucher (Carroll & Graf, $3.95); Letters to Sherlock Holmes, edited by Richard Lancelyn Green (Penguin, $6.95); The Further Adventures of Sherlock Holmes, edited by Richard Lancelyn Green (Penguin, $4.95). None of these books is part of the Holmes canon -- the Sacred Texts -- but each will appeal to admirers of the world's foremost consulting detective. Anthony Boucher offers a comic puzzle, in which a movie company planning to film a Sherlock Holmes story starts receiving mysterious notes from a group calling itself the Baker Street Irregulars; murder and hugger-mugger ensue. Richard Lancelyn Green's two collections provide, respectively, a selection from the amusing, often child-like letters actually sent to 221-B Baker Street, and an anthology of Holmesian pastiches by several hands.

Raffles: The Amateur Cracksman, by E.W. Hornung (Leete's Island Books, Box 1131, New Haven Conn. 06505, $8.95). What Holmes and Watson are to detection, Raffles and Bunny are to burglary. (Indeed, Hornung actually became Conan Doyle's brother-in-law.) Here, gathered in one volume, along with George Orwell's famous essay, "Raffles and Miss Blandish," are all the stories about the gentleman- thief. The chronicles begin when Bunny, having lost all his money at baccarat and written bad cheques to the other players, returns to the casino to shoot himself. Raffles intervenes and recruits the hapless young man into a life of upper-class crime.