Microsoft Press has been distributing some of their books (pretty latest editions) in PDF format for a while (see below for link). They have announced that they will be releasing those books, DRM-free in electronic formats that fit nicely on ebook readers like the Kindle, Sony Reader and other tablets.
PDF formats on these devices, although readable, only gives you the option of zooming but the flow of the text is not very pleasing. If you’ve had a chance to try this out you would know how frustrating this can become. I really look forward to those formatted books in the near future.
http://blogs.msdn.com/b/microsoft_press/archive/2011/03/03/ebooks-list-of-our-free-books.aspx
PS: Most libraries in the US have also started lending out books in EPUB format.