Long time SSC participant Grant Fitchey has published a great book on reading/understanding execution plans. I finally had the chance to read it last week (on vacation) and was pleased with it, he did a great job covering the concepts and I noted that he had the answers to many of the common questions I hear from students. In particular I like the extended coverage of cursors (not commonly used, but they still happen) and XML within query plans (which I use rarely and will be re-reading that chapter). Dissecting SQL Server Execution Plans is available at Amazon, and Red Gate has also been offering it as a free PDF download (check their site). I'll be adding it to my list of recommended reading for those that train with us in Orlando.
These are unsolicited/unpaid comments - no agenda other than recognizing a book that will be valuable to many.