The authors behind the the awesome BDC MetaMan tool have written a book:
SharePoint 2007 Developer's Guide to Business Data Catalog
A got my hands on an early copy via the MEAP and I’m about 3/4 through. If you're new to the BDC, it will be a great reference and it will get you up to speed ASAP. If you have been working with the BDC for awhile now, then it will be a complete resource you can refer to as necessary. Either way, it’s definitely worth a look, it’s the only book on the market dedicated to the BDC.