DES MOINES -- Over the years, authorities found Stephen Carrie Blumberg snooping suspiciously in library after library and caught him with piles of stolen books. But it took a paid informant to uncover what the soft-spoken loner was getting away with.

The informant led FBI investigators last spring to Blumberg's house in the southeast Iowa town of Ottumwa, where they made the nation's largest-ever seizure of stolen rare books and manuscripts, a 19-ton cache valued at up to $20 million.

Blumberg, 41, goes on trial Wednesday on charges of interstate transportation of stolen property, conspiracy and possession of stolen property. The cache of 20,000 books and papers included a Bible from 1480 and three shelves of priceless materials from 1450 to 1500, an era known as the incunabula, a period following the invention of the Gutenberg press.

Blumberg has had nothing to say since his arrest March 20. Two Minnesota men who pleaded guilty to interstate transportation of stolen property and the informant, who was paid $50,000 by the FBI for evidence leading to the treasures, have agreed to testify against him. Blumberg, who underwent psychiatric tests at a Missouri prison hospital after his arrest, has a history of emotional problems and scrapes with the law spanning two decades.

His record includes a 1974 accusation of theft in Fort Lupton, Colo., after his car was found stuffed with books from universities in Iowa, Nebraska, Minnesota and Colorado. A judge later dismissed felony theft charges in that case.

In 1988, Blumberg, who used the alias of Matt McGue, was fined and placed on probation after pleading guilty to trespassing and possessing burglary tools in the closed rare-books section at the University of California at Riverside, authorities said.

The paid FBI informant, Kenneth Rhodes, described himself as a longtime friend of Blumberg's and lived with him on and off in the Ottumwa house for about a year before the March raid.