Along with shipping pallets, wooden cable spools is another one of those items you can usually find for next to nothing on Gumtree or at timber yards. Cable spools have limited repurposing potential because of their shape, but they make excellent coffee table bookcases.
Crafting weblog Pink Stitches decided to turn a free wooden cable spool into a bookcase by painting the spool, adding four casters so that it could be easily turned around to access the books, and added several painted wooden dowels to support the books. The author preferred to keep a rough weathered look on the top of the rolling bookcase, but you could always cut a piece of plywood for a new tabletop or fill in the gaps with wood putty before painting.
For full details and step-by-step construction photos see the link below. If you have an extra cable spool on hand, you could build your own cat cave too.
Spool Bookcase (Tutorial) [Pink Stitches]