I really liked the Kaplan books myself. Powerscore would be a close second. My only qualm with powerscore is that it is overly eager to fit things into specific categories. Categorization should simply be pedagogical tool to help a student remember items. Kaplan does a nice job of balancing this with the practical application of techniques/methodology. (fyi - I really disliked the Princeton Review for LSAT prep)
I'm personally not a huge fan of the initial diagnostic test. It seems like a little bit of a waste. Regardless of your intellectual abilities, you will have no idea what you are looking at, let alone how to deal with it efficiently.
My advice is, in your free time before you start prep, read up on the test. Get to know what, generally speaking, it is all about. Maybe even start exposing yourself to some methodology. This way, when you jump in to actual prep, you will have a nice foundation of work from.