The cases are joined together inside and across the top. They can also be bolted to the ground for extra security. These are simply put back-to-back to double the shelf space. Messy pile of maps atop shelves, optional.
Here's a better look at the short bookcases that will form a u-shape within the room of larger bookcases. The photo does not really show how large the room is, but it leaves plenty of room between the large cases and small for a generous pathway between the shelves. As the small cases are back-to-back, we get the benefit of space both inside, and outside the centre path.
Every couple weeks we're set to buy four more bookcases to complete a section. This way, we aren't tripping over boxes of unassembled shelving, and we don't feel that we're quite breaking our budget either.