The arithmetic of bookshelves

My study is twelve feet (144") long. Currently the long wall is occupied, from left to right, by a 36" full-height oak bookcase (the one in the icon, actually), a 24" wide stack of computer boxes, and three 28" half-height bookcases. That's 39 shelf-feet, (not counting the two full-height, 36" units on the other walls) and it isn't enough. The computer that came in the boxes is over five years old, so there's no huge problem about dumping them. The small bookcases have been with me a long time, but they're just cheap white particle-board from Habitat, rather battered by three moves.

( Bookcase noodling )