Bookshelves are always getting a little bit out of line with each other. This bracket keeps them together and is relatively discrete. It can be attached to the front and back of Ikea's Billy bookshelves at the bottom. It can be attached elsewhere if it's needed.

Screw holes are offset between the two sides, so the screws will not collide. I printed with PLA @15% infill. Do not overtighten the screws though – without solid infill, these can crack. TPU might work out better as a material.