Since I used to work in a bookstore, this bugs me. It's untidy, it's hard to browse, and it's not alphabetized (the horror!). We considered getting another bookshelf, but we don't really have room for it since our apartment is so small, so that's a project for another day when we move somewhere larger. So when I read Harry Potter and the Methods of Rationality and I ran across the idea of double-shelving books and propping the back layer up on boxes or risers of some kind, it seemed like the perfect solution. (By the way, if you aren't reading that fic, stop what you're doing and read it now.)
The Potter-Verres household is far from the first to come up with this - apparently people use two-by-fours, bricks, the boxes that rolls of cellophane or tin foil come in, anything that's long, wide, and solid enough to support a number of paperback books. I left the house intending to get a bunch of two-by-fours and sweet-talk someone at the hardware store into cutting them down to size for me. Instead I poked around the dumpster by the gardening section and found a few of those long planters for seedlings, which seemed to be the right length. (I grabbed a few cardboard boxes just in case it wasn't enough; as it turned out that wasn't necessary.)
I wiped them down when I got home, because dirt prevents tape from sticking (also dirt and books are not a great combination). I raided the recycling containers for our building, got a bunch of newspapers and junk mail, and stuffed the planters with as much paper as they could hold. (Planters are made of fairly thin plastic and I needed them to be as densely packed as possible in order to hold up a few dozen books.) I sealed up the containers with a whole mess of masking tape, folded up a scrap of cardboard to fill in the last few inches since the planter wasn't quite long enough for the shelf, and voila!
Now it's ready for books:
Since I wasn't sure how structurally sound masking tape would be long-term, I turned the boxes tape-side down and stacked books against the plastic side. YMMV.
It took two of these to get our rampaging collection under control, but it looks much better now.
It looks better when the front shelf is mostly smallish paperbacks, though. It works less well when there are trade-sized paperbacks or hardcover books in the front, because it's harder to see the back row. (This could probably be solved by elevating the back row some more.)
"Hey, you don't need this box, right? Is it cool if I play with it instead?"