I don't know if you've seen these before, but national geographic used to make sleeves to hold their magazines (or they still do maybe).
Well I ran across them at half price books for 25 cents in the clearance section and immediately thought, "I can use these in my study as hidden storage." You know CD-roms, computer booklets, discs, etc. I bought every one they had and put them in my study bookcases. I then went to a second location and found more for 50 cents each. Still a great deal.
You can call ahead to see if they have any vs. driving around. I see them often at resale shops and estate sales as well. Easy thing to collect over time as you spot them.
Hope this idea helps you create more storage in your home :)