Kindle is not just a book reader. Thanks to its wireless connection, it’s a very convenient reader for many types of content: books, newspapers, magazines and blogs, not to mention Wikipedia and the web in general.

But its main focus is books, and as a book reader, it is superb. Within minutes, you cease to notice the device — what higher praise can be given? Yes, we want more type and layout options for book designers to create beautiful e-books; yes, we want a higher resolution display to make that type even crisper; and yes, we want color. Even with all its advances, electronic readers are in their infancy. The printed book has had over five hundred years of evolution to reach its current state of usability; it’s amazing the Kindle comes anywhere close.

But the Kindle does things no book can do: fully searchable content, including your annotations; hundreds of books in your pocket; continuously-updating weblogs, with no computer needed; almost everything backed up to Amazon’s servers, with no configuration needed.

Kindle is clearly aimed at the sort of book buyers who save their books to re-read, search, or use for reference — voracious readers with sprawling shelves and stacks of books, the readers who built Amazon’s business from an idea to the largest book retailer in the world.

Kindle excels in part because Amazon has designed the entire user experience (like Apple’s iPod and iPhone), in part because of Amazon’s partnership with content publishers, in part because of its connectivity… but mostly because when you read a Kindle, you forget you’re using an electronic device.

It’s just a book.

A really, really flexible one.