- Microsoft Exchange Server 2007 Unleashed by Rand Morimoto, Michael Noel, Andrew Abbate, and Chris Amaris (Paperback - Jan 23, 2007)
- Microsoft Exchange Server 2007 Administrator's Pocket Consultant (Pro Administrator's Pocket Consultant) by William R. Stanek (Paperback - Jan 31, 2007)
This, of course, includes documentation on Exchange 2003 as well.
Personally, I have used the TechNet documentation extensively in preparing and planning for a migration to 2007. I feel that it's a great place to start if you're looking to just dig in and do some solid reading on the product. Check it out and I think you'll agree!