Every would-be writer intent on a first novel yearns for two things: an uninterrupted stretch of time and no worries about expenses. But being locked up for 10 months in a 17th-century Swiss dungeon seems like an extreme solution. Paul Erdman, an American financial whiz imprisoned in 1970 and charged by the Swiss government with bank fraud, passed his days in the pokey knocking out a mystery-adventure novel set in the arcane world of international banking which he had so recently left. He called it The Billion-Dollar Sure Thing, and it sure was. The book rode the best-seller charts for six months and so far has sold two million copies in 16 languages.

Erdman jumped $90,000 bail in Switzerland last year, moved to San Francisco while awaiting trial (he was sentenced in absentia to nine years) and has now made another killing in the literary marketplace. His second novel, The Silver Bears, sold 50,000 copies in the first month and raked in $500,000 in paperback and movie rights.

For the elegant looking, 42-year-old Erdman, fiction is very much a second career. After taking a Ph.D. in economics at the University of Basel in Switzerland, he worked for the European Coal and Steel Community and the Stanford Research Institute. At the age of 31, he had the bright idea of opening an American-owned bank in Switzerland that would combine go-go management with advantageous Swiss banking regulations. The bank's assets rocketed from $3.1 million in 1966 to over $49 million by 1969, when the United California Bank, one of the 15 largest in the U.S., bought up Erdman's booming institution for about $12 million. He continued to run it.

From then on its corporate history reads like one of Erdman's plots. The bank's commodities department took a beating in silver when the price dropped. To recoup the loss, it bought, he says, about "one half of the cocoa in the world." When cocoa went sour, the bank lost somewhere between $40 million and $66 million.

Swiss authorities, who regard their banks as inviolable as the church, jailed Erdman and six other bank officers. At the time Erdman sheepishly owned up to some fault in the fiasco. But now, socked with a $50 million lawsuit from United California Bank, he blames his former underlings.

Meanwhile Erdman passes the time relaxing with his family at their isolated Marin County retreat near San Francisco, promoting The Silver Bears, and writing a new novel with the grim working title of The Crash of '79. It is an event he claims could be touched off by the Arabs, "a thin but well-educated elite controlling the biggest pile of dough in the universe."

Now and then he gives lectures on such "academic" financial topics as the worldwide economic situation. "People want to grasp what's going on so they can manage things themselves instead of relying on their dummy stockbrokers," he says.

Aside from cashing royalty checks, though, Erdman says he is finished with the funny-money game. "It's a world of strong men wielding power in terms of money. It is filled with people totally devoid of any humor. I like being responsible only to myself."