Google has added an e-books section to the web version of the Android Market, but unfortunately, the purchasing process is not very smooth. I tested the process by ordering a book, and as you can see in the screen shot, there was no immediate download of the book to my phone, such as you can do when ordering books for the Kindle or Nook apps. It does not appear as though books are pushed to a phone over the air like apps are when you purchase them.
Books purchased from the Android Market can be read using the Books app for Android, and similar apps exist for the iPhone and iPad. There is also a web app for reading books, and curiously, the book I purchased has not yet appeared in the web app either, which makes me wonder exactly how the books are being provided.