Another notable growth category shown here is Medical — Though its share of all apps is still small, it’s the third fastest-growing app, behind books and travel.

Here’s Lorica’s text on books:

Measured in terms of number of unique apps Books remains the fastest-growing category: during the week ending 4/12/2009, 11% of the apps in the U.S. store were in the Books category. Books has surpassed the Utilities category and may soon overtake the Entertainment category. Over the past week, Kindle for iPhone was the top app in the Books category.

Here’s the whole chart from Lorica’s article, with all 20 categories of apps:

Thanks to Twitter stars @adamhodgkin and @mtamblyn for highlighting Lorica’s article.

Writing about the Kindle, Jakob Nielsen notes the problem with non-linear content:

The usability problem with non-linear content is crucial because it indicates a deeper issue: Kindle’s user experience is dominated by the book metaphor. The idea that you’d want to start on a section’s first page makes sense for a book because most are based on linear exposition. Unfortunately, this is untrue for many other content collections, including newspapers, magazines, and even some non-fiction books such as travel guides, encyclopedias, and cookbooks … The design decisions that make Kindle good for reading novels (and linear non-fiction) make it a bad device for reading non-linear content.

Later in the review, Nielsen broadens his comments to eBooks more generally. In addition to the issue of linearity, he also mentions that books that depend on pictures are problematic:

11 years ago, I wrote that electronic books were a bad idea. Has Kindle 2 changed my mind?– Yes — I now think there’s some benefit to having an information appliance that’s specialized for reading fiction and linear non-fiction books that don’t depend on illustrations and don’t require readers to refer back and forth between sections.

Paul Biba, in comments on using a cookbook on the Kindle, says:

The concept doesn’t work. This is not the Kindle’s fault, but the fact that some things are just not meant for an ebook format. When using a cookbook one likes to flip through it browsing for recipes. You look at one, go back and compare it to another … see if you can’t combine the ingredients of [recipes] … You simply can’t do this flipping back and forth with an ebook … Going back and forth from the table of contents to the index is a time-consuming process. The ergonomics of the whole thing is just not set up for cooking and recipe browsing.

This is really the first time I have come across a complete failure of the ebook medium. I can’t see how it is possible to make any change in the hardware that would alleviate the problem. There is simply no substitute for flipping pages and marking them with bookmarks … The ebook format is, by its nature, linear and this linearity is not adaptable to serious cooking.

Here’s the text that accompanies the picture : “Comparing comic books on the Kindle and the Kindle iPhone app. The quality isn’t great since you can’t zoom in (which is a problem for reading text), but the potential is there. (Credit: Nicole Lee/CBS Interactive)”

And here are other comments from Lee’s story on color …

… there is one potential advantage the iPhone has over the Kindle, and that is this: Color. Why would you want color in an ebook? Why, for comics, of course. I’m a big comic book fan, so I went looking for comics in the Amazon Kindle Store to see how it would look on the new Kindle 2 with its 16 shades of gray. There aren’t a lot of choices out there, so I just downloaded a few samples to check them out. The results are not that great, sad to say. Each comic page is considered an image, so it’s a little slower to download. I was fine with the gray shading, but the comic format does not quite fit the size of the Kindle 2’s screen. Smaller format books like manga digests look a little better however. Still, navigating the pages is a pain. There’s no way to zoom in and out of panels, and if I wanted to enlarge the images to full-size, I had to do so for each page. Plus word balloons are almost impossible to read since I can’t zoom in.

I downloaded those same comic samples to the Kindle application on the iPhone. I still couldn’t zoom in, and it’s still hard to read the word balloons. But I was awed and amazed that they arrived in full-blown color. Yes, I couldn’t read any of them, but it gave me a small glimmer of hope that maybe some day there’ll be a way. Until then, I guess I’ll have to live with the individually-sold iVerse comic applications.

It’s interesting, of course, that the first format that shows the value of color e-books is comic books — But, the implications are far-ranging for illustrated books in general, such as medical and science textbooks in which illustrations have an important place.