We had tons of books too when I was growing up, and I have to say that having so many amazing books around helped shape who I am today.
I sometimes buy books from second hand book sale websites - it's a great way to save on buying reading material, and you're basically giving a second lease on life to every book you buy (and it's great for the environment too!)
Here are a couple of sites I've purchased books from before:
http://www.betterworldbooks.com/ - they have a great collection of even older-print encylopedia, I managed to rebuild a whole Childcraft collection printed 1980-81!
http://www.thriftbooks.com/Hope that helps!