Materials: 4 steel plates, 4 right angle plates, a few screws and screwdriver
Description: The book collection grew beyond the half height Billy bookshelf, then beyond two, then at four Billys it just started getting awkward.
I originally doubled these up without any hardware, but after moving to SF and not knowing what an earthquake was like, I had to make it a little more robust.
1. Assemble your Billys 2. Use the right angled plates to combine to of the Billys into a nice right angle. 3. Mount a Billy (upside down for symmetry!) on top, and use some flat plates one the side (see photo). 4. Do the same for the final fourth Billy.