But there are many options. Fictionwise.com, a bookseller that supports many formats, sells some new novels as multiformat packages that let users choose among 10 readers from companies like Microsoft, Franklin or Adobe. Microsoft’s Reader (www.microsoft.com/reader), for instance, works only with versions of Windows running on PCs or PocketPCs. Adobe distributes a PDF reader for most major platforms, including cellphones, which runs the Symbian OS found most commonly on Nokia phones.

The basic readers are available free, but some companies offer enhanced versions for a price. The “pro” software from eReader.com costs $9.95 and offers a bundled dictionary and more sophisticated features like auto scrolling. There are also open source packages like Plucker (plkr.org) available free. Many people install several readers so they have at least one that matches the format of the book they want to read.

Advanced cellphones with large screens, like the Palm Treo or the Apple iPhone, can usually display e-books formatted in basic standards like the PDF or HTML, but their controls are not optimized for long texts. (The drawback: many cellphones have bright displays that drain batteries faster.)

The iPhone also has a screen that is larger than most other cellphone models, and what is on the screen is displayed more crisply. But it has no custom book reader and no easy way for developers to rewrite their reader package for the phone. The company is deliberately pushing programmers to package their content for Safari, the Mac Web browser bundled with the phone. Users who want to read books on their iPhone need to choose HTML or PDF formats.

It is only a matter of time before users create tools specifically for the iPhone, said Michael Hart, the founder of Project Gutenberg, a repository of e-books no longer under copyright protection. “The iPod was only out one week before we had e-book readers running on them, so it will be no surprise if there are multiple sets of programs, readers and formats for the iPhone,” he said.

Indeed, Stephen Pendergrast, the chief technology officer at Fictionwise.com, said his company is trying several formats, including narrow PDF files with margins set to the screen’s width.

Prices for digital books are often lower than those of printed versions, but the difference may not be as great as people assume. “On the e-book side, the author gets a higher percentage,” said Mr. Pendergrast. A bigger cut for the author means less for the publisher, and so the publisher charges more to cover costs. The price of a best seller at Fictionwise is $6.79 with no shipping charge when you download it from the site. Amazon typically charges $7.99 for a paperback best seller and requires an order of $25 to qualify for free shipping.