Sales of electronic books fell by the greatest drop since e-books were first marketed.
E-book sales had been growing robustly, except for a small dip in 2013.
But in the first five months of 2015, sales were down by 10% compared to the same period in 2014, according to the Association of American Publishers.
The latest figures showed US e-book sales (not including educational texts) of $610m (£400m).
Sales of printed books also dropped, but at the much slower rate of 2.3%.
Readers took up the e-book habit with alacrity over the past few years, but their share of the market has levelled more recently, accounting for 23% of the market in 2014, according to the association.
"In the early years of e-books, the percentage growth from physical books to digital books was rapid.
“Now that e-reading is fully integrated into a consumer's choice of options, the market has begun to stabilize," Tina Jordan of the association said.
The association tracks monthly sales of 1,200 publishers and has monitored e-book sales since 2002.
