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.

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