Step 2: Minding the Gap
Now that you have your bookcases you have to start making some aesthetic choices. In order to have decent clearances I opted to space out th bookcases and use 5.5" boards as trim.
In this pic you can see the bookshelfs are fairly spaced apart, but the boards help to make them look like one piece.
I used angle brackets and put in a board across the top. this helps to stabilize the shelves as well as provide a top pivot point for the center bookcase (which I'll go into later)