Step 1: Measure
We used the Borgsjo Bookcase in white from Ikea for this project. The bookcase is 29-1/2" wide. 5 bookcases fit on our 12'11" wall. I measured, measured again, and even sketched a to-scale rendering to make sure we would like the finished look using this size of bookcase.
Note your ceiling height at this point as well. Ours are only 8' and these bookcases, which are on the shorter side at 71-1/4"H, fit it well. If you are working with taller ceilings, you may consider choosing a taller bookcase.