Lack of shelvage

I love books! Spent the day trying to clear the piles of books that have accumulated around the house. Why do I never have enough book shelves? I just bought two from Ikea, but it's still not enough, and K wasn't crazy about even those. I've been sorting them out too, trying to classify them into groups that mean something to me, so that I might be able to find them in the future. So far, in vague volume order, I've got:

• S.F. (by far the largest group)
• Non-S.F. fiction
• Space history
• Non-space history
• Popular Science
• Software textbooks
• Humour (Mostly Calvin & Hobbes, Dilbert and Peanuts)
  
• Non-computer textbooks
• Travel (I'm not sure about this one - but where else to put Bill Bryson?)
• Philosophy & religion
• Other (all kinds of rubbish in here)

Sometimes it's hard to find where a book should go. Perhaps the bin beckons for Erich von Daniken? K reckons I have too many books. Maybe I should instigate a policy of chucking out a book before I buy a new one. Mentioned this to WTH before xmas - he thought I was crazy. I need a huge 'wall shelf' like he has, but good solid walls are few and far between in our house.