I think the SAP Pratical Workflow 3rd edition is a great book 
. I also like the SAP Web Client book which was very useful in my day to day work. The SAP CRM Middlweare Optimization guide which is an older out of print title, also helped in solving quite a bit of problems for me. If I was limited to only one SAP Press title as my favorite, I guess I would have to choose SAP CRM Technical Principles and Programming, as that book resonates with me on a personal level 
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Take care,
Stephen