They're by period and style.

[ Which is super nerdy and makes Booker kind of blush a little. Who organizes their books by period and style?

He gets up from the bed and goes over to Nicky. ]

I can make room so you've got a shelf for your own. You don't have to throw them in with mine.

[ And his weird organizing system.

He'll have to rearrange his books a bit but that was fine to let Nicky have some space. ]