There's little question that the latest Kindle 2 hardware is much improved over previous versions, but it still isn't quite an easily pocketable, ubiquitous device: it's purpose-built for reading. Although it's portable, I'll wager people are far more likely to have a phone on them at all times. And that's where Amazon's new Kindle for iPhone app comes in.

"We are excited to bring the new Kindle application to Apple's App Store and think customers are going to love how easy and fun it is to read their Kindle books on the iPhone and iPod touch, said Amazon Kindle vice president Ian Freed in a statement. "Kindle for iPhone and iPod touch is a great way for customers to catch up on their current book wherever they are, like in line at the grocery store or between meetings."

The application—a free download from Apple's App Store—is simple to use and makes a great companion for Kindle owners. But owning a Kindle is not a requirement. Books can be purchased via Amazon's Web-based Kindle Store and synced to an iPhone. Despite the fact that the iPhone has a smaller, somewhat lower-resolution screen than the Kindle's e-Ink hardware, reading on the iPhone can actually be quite enjoyable. But it does have some serious usability caveats.

One you register your Amazon.com account, you can access all your previously purchased books.

First, you link to your Amazon.com account, so you'll need to get one if you don't have one already. If you are a Kindle user, books you have already purchased will show up under "Archived Items," and selecting books here will allow you to sync them to your iPhone. Titles synced to your iPhone will show up in a list on the home screen.

If you haven't already purchased books, or you want to buy new ones, this is where it gets tricky. There is no in-app capacity for purchasing books. This may be due to a stipulation in Apple's iPhone Developer agreement, which says that programs cannot provide additional content through distribution methods other than the iTunes Store without prior written consent from Apple. Most other e-book readers for the iPhone that offer the option of purchasing books, like Stanza or eReader, get around this by routing users to an e-book store via an embedded WebKit view. So far, this approach hasn't been rejected by Apple.

Just because you can buy books via MobileSafari doesn't mean you should. But, it can be done—an unabridged copy of Stephen King's The Stand is being sent to my iPhone via WhisperSync?.

The current version of Kindle for iPhone is a little different. Clicking on the "Get Books" button on the Home screen instructs users to got to Amazon's Kindle Store via a computer for "the best shopping experience." And they aren't kidding; while there is a link that will open the Kindle Store in MobileSafari, browsing and buying books this way is just plain frustrating. The Kindle's own integrated buying system is far simpler in comparison.

Apple presumably has this restriction in place so that developers don't abuse the App Store system, giving away free apps on Apple's dime and then selling content elsewhere. Perhaps Amazon can build an iPhone-browsable version of the Kindle Store and display it via an embedded browser, or better yet, perhaps Amazon and Apple can come to some sort of agreement to allow in-app purchasing.

Reading, on the other hand, is pretty straightforward. Books can be navigated via a hyperlinked table of contents. Flipping from page to page uses the same swiping gesture as most other e-book readers for iPhone, though it doesn't allow tapping on the edges like most of those others do. Resting a finger on the screen will bring up a navigation overlay, which allows setting a bookmark, jumping to the table of contents or other bookmarks, changing the type size, and syncing your location within the book with your account. There is also a slider that allows jumping ahead by arbitrary pages, and a button to jump back to where you were previously.

I'm banking on The Stand lasting through several weeks of commuting. The text is set pretty small, but it helps avoid large gaps and "rivers" due to the justified text.

While the other functions are fairly obvious, the syncing is quite handy if you move from Kindle to iPhone. It will sync your last read page, as well as any new bookmarks, from one device to the other via your Amazon account. So, if you were reading at home, say, and then later wanted to pick right back up where you left off while out and about, syncing makes that happen.

Visually, Amazon has done a pretty good job of making text readable. Any books that have images will show those inline, and books formatted with color will display in color on the iPhone—something the Kindle's black and white screen can't do. The only major gripe we have here is that text is always displayed justified. There is no option to display text ragged-right, which is generally far easier to read, especially with narrow columns of text. The justifying leaves a lot of variable-width gaps between words, and it can make for some uncomfortable reading.

Moreover, there is very little control over how text is displayed. Most other e-book readers allow for adjusting typeface, leading, justification, background color, and other settings. Kindle for iPhone has one typeface, one (less-than-ideal) justification setting, a few type sizes, and little else. Kindle for iPhone also doesn't allow for landscape viewing. These limitations likely reflect an effort to keep the reading experience somewhat similar to the Kindle itself, but I'm not sure iPhone users will be able to overlook all of them.

Kindle for iPhone is a great addition to the stable of e-book apps available for the platform. The competition will have a hard time keeping up with Amazon's catalog of content, and for current Kindle owners it makes a great companion. If Amazon can add a more control over text display and find a way to offer browsing and buying content directly from the app, it might become the go-to e-book reader for iPhone.