Looking for a guide to best classical recordings

I'm a rocker, or so I like to think. So as you might guess, my knowledge of classical music is minimal. I am aware, however, that just having a recording of a certain piece is not good enough; that there are often "definitive" recordings of various pieces. I'm interested in some sort of guide to the "best" recordings. I think I saw some sort of Penguin guide once upon a time. Any of you have recommendations on what specifically to look for?

Thanks in advance,

Nick