"Jimmy Carter has his brother Billy and Fritz Mondale has his 'billy brother," is the sly way Lester Mondale describes himself.

But although he lives in a rural home made of logs near the junction of the St. Francis River and Brewers Creek in southeastern Missouri, the vice president's half-brother is no hick. He's an erudite retired Unitarian minister long active in the American Ethical movement. And at age 74 Lester Mondale has just completed a potentially commercial novel that is, well, racy for a man whose previous seven books were narrowly focused philosophical and theological tomes.

"I suppose it is utterly ridiculous at my time of life to be trying to write something like this, but it's my third attempt at fiction and Scott Meredith seems quite excited," says Mondale.

Indeed, an agent at Meredith's Manhattan literary agency (clients include Andrew Young and Judith Campbell Exner) says Mondale's novel titled In Erosland, has "good commercial" possibilities.

The plot: Pastor Goodfellow Hanson, a dashing man in his early 40s, is targeted for seduction by an attractive married woman in his congregation. The pastor's wife is a domineering sort who wants to run her husband's church and, as Mondale puts it, "is anything but sexually responsive."

Mondale vows the novel is not autobiographical, though he says the incidents and characters are drawn from his life's experience. And the premise for the action - a woman developing a crush on her spiritual counselor - is all too common in reality, says Mondale.

Is the sex explict? "Pretty close to it," allows Mondale, who quotes this praise from a letter by a publisher who nonetheless decided not to bid on the book: "It is a relief to come across a novel which treats the territory of eros as the region of the mind, in place of the heavy breathing we have had to listen to ad nauseam in recent years."

What if success should come to Copperhead Cliff, as Mondale calls the home he and his wife Maria built with the help of neighbors in the Missouri woods? How would he like hyping his book on the talk-show circuit, this man who lights his wood stove every morning at 6 and reads about his famous younger half-brother when neighbors pass along copies of newsmagazines?

mondale admits he hasn't really given any thought to commercial success, but says, "I'll do anything legitimate for the fun of it." And, like a pro, he says if he sells this manuscript, he's ready to begin a sequel.

Footnote: Lester Mondale had already left home when his father and stepmother had a baby named Walter. Not until later did the two come to know each other, and Lester and his wife, a mathematician, worked on both of Walter's Senate campaigns in Minnesota. Lester hasn't seen the vice president since Inauguration Day, but the veep's staff stays in weekly touch. Among other reasons, Lester and his wife complied a 175-page book tracing the Mondale family's genealogy; when people write the White House claiming kinship to Walter Mondale, it's Lester who checks it out. And when Mondale made a trip to Canada, Lester upon request found details of Mondale forefathers who lived there, which the vice president used in his speeches north of the border.