Microsoft Press Books, while not always the best in their catagory are certainly useful. One feature I've come to really appreciate is that most of their books include a CD-ROM containing a full copy of the manual in eBook format, typically PDF.
This isn't to say other publishers like Sybex and O'Reilly don't do this, but it seems far more common with MS Press. Anyone who's lugged a 1310 page tome around with them can appreciate a searchable PDF.
For the last few months, my first search on a given tech subject is for a MS Press book with an eBook.