Library stacks ahoy:
They're not bolted together or to the wall yet, and obviously the shelves haven't all gone in, but you get the general idea. As you can see, the stacks go three across + a fourth bookcase that "caps" the end (instead of just having a blank space where the bookcases come together). That fourth bookcase is deeper than the ones used to make the stacks, so that it carries its own weight. The cap idea was inspired by arrangements at the Strand.
Although you can't see them in the photo, there are track lights running between each row of bookcases.