I've been seeing several people blogging about the free Microsoft Press E-Book offer. All you need to do is register and you get access to 3 e-books from Microsoft Press that you can download in PDF format.
If you pay close attention to the details for each book, you will notice that these aren't full copies of the books. The only one that appears to be a full copy, based on the listed chapters and the actual table of contents in the PDF, is Introducing Microsoft LINQ. The other two e-books only contain two chapters each, even though the table of contents for both of these list 8 chapters.
While the idea of offering free e-books is great, don't advertise these as full books unless they really are.