New copies of books published by Macmillan were unavailable in ebook form on yesterday, apparently part of the biggest rift yet in the ongoing dispute over prices.

An official with knowledge of the dispute said the two sides were in talks, but would not say why took such a public step. The official asked not to be identified, citing the sensitivity of the talks.

Macmillan and other publishers have criticised Amazon for charging just $9.99 for best-selling e-books on its Kindle e-reader, a price publishers say is too low and could hurt hardcover sales.

Some of Macmillan's new titles include Hilary Mantel's Wolf Hall, a novel about political intrigue during the reign of Henry VIII that won Britain's prestigious Booker Prize, and Andrew Young's The Politician, an aide's account of the sex scandal involving presidential candidate John Edwards.

For its part, Amazon wants to keep a lid on prices as competitors line up to challenge its dominant position in a rapidly expanding market.

The most talked about challenger is Apple, which has just introduced the long-awaited iPad tablet computer and a new online book store modelled on iTunes.

Apple chief executive Steve Jobs, in an interview with the Wall Street Journal, suggested publishers may offer some e-titles to Apple before allowing them to be sold on Amazon.