Well I'm not very well read in this area, but one book I would recommend is "1001 Classical Recordings You Must Hear Before You Die", does what it says on the tin, good for classical newbies (like me!) and aficionados alike. It's a great reference and buyer's guide, it goes right from the 12th century to the 21st, everyone from Hildegaard Von Bingen to Karlheinz Stockhausen. Guaranteed to create a few arguments among classical buffs!