With the free application, available on Apple’s application store, consumers can synchronize Kindles with iPhones and iPod Touch, giving those users access to more than 240,000 books available on Kindle, Amazon said in a statement on Wednesday.

With the “Kindle for iPhone and iPod touch” application, a user can read a few pages of a book on the Kindle or its newer version, Kindle 2, and continue reading from where they left off on the iPhone or iPod Touch.

Users can shop for books on their Kindle or online and wirelessly transfer the books to the iPhone or iPod Touch. They will also be able to access all previously bought Kindle books, adjust the text size or add bookmarks.

Though a tiny fraction of Amazon’s business, the Kindle draws strong interest from investors and gadget lovers, amid speculation that the device might eventually be enhanced to compete with devices like the iPod.

Last month, Amazon unveiled a slimmer Kindle 2 with more storage, but kept a high price tag at $359 that is likely to limit its appeal to a mass audience.

Analysts say Amazon may not want to push the device too quickly into the market, so it can preserve a much higher revenue base from selling printed books and avoid direct competition with companies like Apple.

The online retail giant pledged to modify the Kindle 2 so that authors, publishers or any holders to a novel’s rights can choose whether to turn on the feature, which takes written text and converts it to human speech.

“Kindle 2′s experimental text-to-speech feature is legal: no copy is made, no derivative work is created, and no performance is being given,” the firm said in a statement on Friday.

“Nevertheless, we strongly believe many rightsholders will be more comfortable with the text-to-speech feature if they are in the driver’s seat.”

In an editorial titled “The Kindle Swindle” that appeared in the New York Times Wednesday, the president of the Author’s Guild, Roy Blount Jr., took Amazon to task for the function on the new Kindle, which began shipping this week.

The new Kindle can read books aloud, but unlike with audio books, royalties are not paid to authors. Blount argued the technology Amazon uses to turn text into a human voice is quickly improving, and authors need to be “duly vigilant” about this novel means of transmitting their work.

The guild, which is studying the issue, has called the Kindle’s speech function a “significant challenge to the publishing industry.” It has recommended its members bring up the issue of the Kindle when negotiating book contracts.

On Friday, Amazon said rights-owners will be allowed to decide — title by title — whether to enable the function.

Though a tiny fraction of Amazon’s business, the Kindle draws strong, regular interest from investors and gadget aficionados, amid speculation the device might eventually be enhanced to compete with all-rounded, hand-held devices like Apple’s iPod.