Spending on printed books fell by 4% in August, with sales dropping to a seven-year low for the month, according to sales analysts Neilsen BookScan.
Readers spent £107.5m on physical books last month, down 4% on July and down 8% on August 2010, The Bookseller reports.
Book retailers have faced increasing competition from online retailers and from the rising popularity of e-books.
Yet physical non-fiction book sales were up 3% on this time last year, with significant gains for some genres.
Food and drink, biographies and the fitness sector all enjoyed significant gains on last year, according to The Bookseller.
Recent BookScan data reveals that supermarkets, which traditionally do a good trade in holiday fiction, have seen their sales sales down by 12%.
The British Retail Consortium (BRC) described book sales throughout the UK in August as "tough".
Sales of books, particularly fiction titles, were "well down on a year ago", the trade association said.
One title did perform well, however - David Nicholls' One Day, recently filmed with US actress Anne Hathaway.
August's top-selling book was officially Lee Child's Worth Dying For, which sold 120,588 copies.
Technically, though, the combined sales of Nicholls' novel and its film tie-in edition made it the month's most popular title.