You read that right: the Market now includes a totally awesome Books site. To be honest though, none of us is really sure what all has changed, since none of us really used it before - but it doesn't seem like a lot, other than a flipped switch. Let's start with what it looks like:
Notably, there's now a dropdown to sort by genre (to the left of the search box, above). When you select a book (protip: Sherlock Holmes is free... or if you're like Artem: Walden is free.), you're presented with the option to read it on the web or, assuming you already have Google Books set up and synced, on your Android device. In other words, it's the fairly typical web Market interface, albeit with the Web option.
While you can search for books via the web Market, you still need to use the Books app to search from your device. In fact, it appears that the Google Books app itself remains the same, with searches still redirecting to books.google.com.
It seems fairly safe to say that they've chosen to flip the switch today thanks to Honeycomb now being out in the wild. Unfortunately though, the Books section of the Market seems to only work for those in the U.S. - for those in other countries, the shelves are empty for the time being.
If you're in the U.S., head on over to the link below to check it out for yourself.