gordk, i wouldn't worry to much about booking ahead unless you are looking for a 3-4 star hotel in Madrid or Barcelona during peak season. 3 years ago a took a 6 week trip around spain and never once had advanced reservations. we had a lonely planet book with us and once we reached the city/town where we wanted to stay the night we just called around to see what hostals had an open room. many times the hostal owners or people who worked in bares were able to refer us to a nice place to sleep. of course we were visiting several small towns and it was may-june but we never had any problems finding a very decent, clean, and inexpensive hostal to stay in. i hope this gives you an idea of how easy lodging can be in spain if you are laid back and not too caprichoso about it, if you keep this in mind i think the worst that can happen is quite encantado.
megan