I finally have the space to finish a project I've been working on which is a study with 3 bookcases. So far, my idea is to have 1 with books that will always be there, such as classic reads, or even an encyclopedia maybe?, or other reference material. Basically, a permanent bookcase whether or not I've read the material. The other two will be rotated in and out of stuff that I'm reading, have read recently or on my backlog before swapping or donating.

Anyways, what's in your "must have" bookcase? Reference, fiction/non-fiction, Calvin & Hobbes even! (Although that's more of a coffee table piece)