A good resource is "Aircraft Buyer's Handbook" by Bill Hemmel.

I also purchased "Used Aircraft Guide" from Aviation Consumer. It's a two
volume set describing the good and bad points of most singles and light
twins and is a great help in deciding what you're looking for. It's $70 but
that's peanuts when buying a plane.

Also check out Aeroprice for price estimation software. VREF numbers are
usually 10% high and not very accurate.