hmmm....could you be a bit more broad???
seriously, people (including myself) have written entire books on the subject. i suggest you start perusing them...but don't buy until you are ready to go. borders and b&b will let you look for free, also your library.
in short...
--if you are wealthy and middle aged...try frommers, fodors, or DK
--if you are middle-aged and middle-salaried...try Rick Steves (he also only covers teh essential places)
--if you are a poor student or just poor--lets go and rough guide can't be beat. lonely planet is OK, but it is all over the place, hard to figure out and often wrong
your best bet is to figure where you want to go and then buy specific guides for that area.
finally....use the web, specificallly
www.tripadvisor.com there forums are amazing.
here on mm....if you get suggestions it will be a wildly diverse mix as we all live here and love it here and have a much more intense knowledge of way off the beaten places that maybe a casual tourist would find boring or at least inaccessible.
good luck!
and if you are travelling after april 07, you can buy my book..Moon Handbooks Spain..it is the best! and it answers specifically your questions...but in 800 pages.
cheers,
candela