I don't usually recommend books as none of my co-workers want anything to do with SQL. That being said I know when i look for books I pretty much go by author reputation. I have Grant's Query Performance book, I'll buy anything by Kalen delaney, Paul Randal or Kimberly Tripp, Itzak ben Gan, Lynn Langit, Joe Sack (I'm not sure if he writes anymore). If I know of the author chances are I follow thier blog and like thier writing style. Most of the books I buy are pretty specialized. I've just picked up Aln hirts Failover Cluster book, General books about SQL are a hard sell for me, I don't need a(nother) book that tells me about CRUD (unless it provides a hideous level of deep details).