It depends on what engine you are using once you are talking about xplans, if you are into SQL Server and you are looking for a great book to reference thats almost all practical "this is how stuff is going to be interpreted", check out http://www.amazon.com/SQL-Server-Query-Performance-Tuning/dp... by Grant Fritchey (the previous book had Sajal Dam)
Additionally, SQLPass puts out a great DVD every year and in the past year or two they had some great talks about how to read xplans, why they are actually lies, and going into things like statistics io, measuring recompile/cpu time, determining if implicit conversions are causing issues, etc etc etc
Additionally, SQLPass puts out a great DVD every year and in the past year or two they had some great talks about how to read xplans, why they are actually lies, and going into things like statistics io, measuring recompile/cpu time, determining if implicit conversions are causing issues, etc etc etc