Jim Hollock’s first book, a true-crime tale set in Pennsylvania, got strong reviews and decent sales when it appeared in 2011. Now “Born to Lose” is losing momentum — yet Amazon, to the writer’s intense frustration, has increased the price by nearly a third.

“At this point, people need an inducement,” said Mr. Hollock, a retired corrections official. “But instead of lowering the price, Amazon is raising it.”

Other writers and publishers have the same complaint. They say Amazon, which became the biggest force in bookselling by discounting so heavily it often lost money, has been cutting back its deals for scholarly and small-press books. That creates the uneasy prospect of a two-tier system where some books are priced beyond an audience’s reach.

It is difficult to comprehensively track the movement of prices on Amazon, so the evidence is anecdotal and fragmentary. But books are one of the few consumer items that still have a price printed on them. Any Amazon customer who uses the retailer’s “Saved for Later” basket has noticed its prices have all the permanence of plane fares. No explanation is ever given for why a price has changed.