Though it's just five months old, Apple's iPad is a certifiable hit, having already sold millions of units and spawning tens of thousands of apps tailored for its 10-inch screen. The tablet has prompted many of its owners to use it instead of their laptops for everything from email and social networking to games and Web surfing.

It's also a very good e-reader, in my view.

Unlike dedicated e-reader devices like Amazon's Kindle, the iPad offers a wide selection of e-reading apps, and I have used several of them heavily to devour scores of books. In particular, I have spent the past few weeks testing the best known of these iPad e-reader apps, comparing their strengths and weaknesses.

My verdict is that none of the three apps I focused on -- which mimic and often interact with dedicated e-readers like the Kindle device -- towers over the others. Each has its good and bad points, and I personallyswitch among them.

First, let me note that this isn't a comparison of the iPad and the dedicated e-readers. It is about software readers on the iPad itself. Some folks will prefer the focused e-reader hardware, such as Amazon's Kindle, Sony's Reader and Barnes & Noble's Nook. The latter devices cost much less -- the base Kindle is now $139 versus the iPad's $499 starting price. They also have longer battery life and are much lighter. But others -- including me -- prefer the iPad's big, bright, backlit color screen to the smaller, gray screens of the dedicated e-readers, and the fact that they can pause periodically in their reading to do so many other things on the iPad without reaching for a laptop.

For this review, I compared Apple's own fledgling e-reader software and store, called iBooks; Amazon's Kindle iPad app; and the newly revamped Barnes & Noble iPad app, called Nook.

Overall, they are more similar than different. Each is free and operates much like the pioneering Kindle device, offeringaccess to an online library of books you already own and an online store to buy more. Each remembers where you left off in your books, and includes built-in search, dictionaries and the ability to enter notes and to highlight text. All also offer the option to search for more information on terms in your books, using Google or Wikipedia.

Apple's iBooks app visually is the slickest of the three. Its library screen looks like a woodenbookcase, and when you turn a page, it curves like a paper page and even shows the text on the other side bleeding through. When you hold the iPad horizontally, iBooks switches to a two-page view with a rounded rise in the middle, like a paper book's binding. The iBooks app is the only one of the three to offer a built-in bookstore, while the Amazon and Nook apps require you to jump into the Web browser on the iPad to shop. This is because Apple charges third-party app developers 30% to make in-app purchases, a price Amazon and Barnes & Noble prefer not to pay. This may be an unfairadvantage for Apple, but it's convenient for users.

The iBooks app also can handle personal PDF files, synced to the app via iTunes on your computer. Neither of the other two apps offer PDFs on the iPad, though Amazon and Barnes & Noble say they're working on it. Also, Apple has harnessed the iPad's accessibility features to allow its e-books to be read aloud, something I couldn't make happen in the other two apps.

But iBooks has some big downsides. The most important is that, being only five months old, it has a smaller catalog than its rivals -- just 130,000 titles, versus around 700,000 for the Kindle app and about one million for the Nook app. For instance, the popular Swedish mysteryseries by Stieg Larsson is absent from the iBooks catalog. And iBooks doesn't offer any periodicals.

Amazon's Kindle app has the biggest catalog of commercial, copyrighted, in-print books -- about 655,000 titles. The Nook catalog of a million books is larger overall, but about half consists of out-of-print books. The Kindle app also instantly displays the dictionary definition of any word you highlight. The others require you to press a dictionary icon to look up a word. And, like iBooks, it was fast at opening books.

The Kindle app also lets you see popular highlighted passages selected by other users, and it synchronizes the last page read, your bookmarks and notes with the Kindle hardware reader and Kindle apps on Windows PCs, Macs, and BlackBerry and Android devices. iBooks only syncs these things to the iBooks app on other Apple hand-held devices, the iPhone and iPod Touch. The Kindle app also can be set to turn pages with the same curved effect as iBooks (but without the text-bleeding effect) and it has a two-page view in horizontal mode.

The Kindle app also lacks periodicals, though Amazon says it's working on this. And the Kindle app, like the Kindle hardware, doesn't use real page numbers, relying on confusing 'location' numbers. The others use page numbers. Also, some books appeared in the Kindle app in scanned, hard to read typefaces.

The Nook iPad app has a big plus: It lets you lend and borrow some titles to and from other Nook users for two weeks. It's also the only one of the three to offer periodicals, though not all are available. For instance, The Wall Street Journal and the Boston Globe can't be downloaded, though the New York Times can.

The Nook also offers more visual effects than the others, including color themes for background and text colors. Also, like the Kindle app, it syncs with Nook apps on numerous other devices, though, curiously, not yet with the Nook hardwaredevice.

But I found more limitations and flaws in the Nook app's basic book functions. For many words, the app lacked dictionary entries the others had, and books loaded more slowly. Also, one book I downloaded on the Nook app had the first few pages missing and another turned out to be a different book from its title. Also, its horizontal view didn't work for all the titles I tested.

In my tests, book prices seemed roughly similar on all three apps. For instance, Jonathan Franzen's new book, 'Freedom,' is $12.99 on each; David McCullough's classic '1776' costs $13.99 on each; and Laurie King's 'The Beekeeper's Apprentice' is $9.99 on all three. Amazon says 574,000 of its 700,000 e-books are $9.99 or less. Barnes & Noble says the 'vast majority' of its commercial e-books are $9.99 or less. And Apple says 75% of its paid books are $9.99 or less and 25% of its paid books are less than $4.99.

Overall, each of the three iPad apps makes the device a fine way to read e-books. Multiple apps and stores -- including many not covered here -- allow choices absent from dedicated readingdevices.

Walter S. Mossberg

• screen [skri:n] n.银幕 vt.遮蔽 (初中英语单词)
• device [di´vais] n.装置；器具；策略 (初中英语单词)
• comparison [kəm´pærisən] n.比较；对照；比喻 (初中英语单词)
• reading [´ri:diŋ] n.(阅)读；朗读；读物 (初中英语单词)
• review [ri´vju:] v.&n.复习；回顾；检查 (初中英语单词)
• ability [ə´biliti] n.(办事)能力；才干 (初中英语单词)
• wooden [´wudn] a.木制的；呆板的 (初中英语单词)
• advantage [əd´vɑ:ntidʒ] n.优势；利益 (初中英语单词)
• convenient [kən´vi:niənt] a.方便的 (初中英语单词)
• computer [kəm´pju:tə] n.计算机；电子计算器 (初中英语单词)
• working [´wə:kiŋ] a.工人的；劳动的 (初中英语单词)
• instance [´instəns] n.例子，实例，例证 (初中英语单词)
• mystery [´mistəri] n.神秘；秘密；故弄玄虚 (初中英语单词)
• series [´siəri:z] n.连续；系列；丛书 (初中英语单词)
• absent [´æbsənt, əb´sent] a.不在的 vt.使缺席 (初中英语单词)
• commercial [kə´mə:ʃəl] a.商业的 n.广告节目 (初中英语单词)
• instantly [´instəntli] ad.立即，立刻 (初中英语单词)
• opening [´əupəniŋ] n.开放；开端 a.开始的 (初中英语单词)
• available [ə´veiləbəl] a.可用的；有效的 (初中英语单词)
• journal [´dʒə:nəl] n.日记；日报；杂志 (初中英语单词)
• background [´bækgraund] n.背景；经历；幕后 (初中英语单词)
• curiously [´kjuəriəsli] ad.好奇地；稀奇古怪地 (初中英语单词)
• missing [´misiŋ] a.缺掉的；失踪的 (初中英语单词)
• tablet [´tæblit] n.碑；牌；匾额 (高中英语单词)
• kindle [´kindl] v.点燃；(使)激动 (高中英语单词)
• selection [si´lekʃən] n.选择；选拔；精选物 (高中英语单词)
• devour [di´vauə] vt.吞食；毁坏 (高中英语单词)
• personally [´pə:sənəli] ad.亲自；就个人来说 (高中英语单词)
• switch [switʃ] n.开关 v.转换 (高中英语单词)
• battery [´bætəri] n.炮兵连；炮台；电池 (高中英语单词)
• offering [´ɔfəriŋ] n.提供；礼物；捐献 (高中英语单词)
• access [´ækses] n.接近；通路；进入 (高中英语单词)
• roughly [´rʌfli] ad.粗糙地；毛糙地 (高中英语单词)
• classic [´klæsik] a.第一流的 n.杰作 (高中英语单词)
• verdict [´və:dikt] n.裁决，判决；判定 (英语四级单词)
• hardware [´hɑ:dweə] n.五金器皿 (英语四级单词)
• binding [´baindiŋ] a.捆绑的 n.捆绑(物) (英语四级单词)
• amazon [´æməzən] n.女战士 (英语四级单词)
• unfair [ʌn´feə] a.不公平的；不正直的 (英语四级单词)
• swedish [´swi:diʃ] a.瑞典人 n.瑞典语 (英语四级单词)
• definition [,defi´niʃən] n.限定；定义；明确 (英语四级单词)
• blackberry [´blækbəri] n.黑莓 (英语四级单词)
• horizontal [,hɔri´zɔntl] a.水平的，横的 (英语四级单词)
• bookcase [´buk-keis] n.书架(箱) (英语六级单词)