What books you keep at the school will depend on a) your curriculum, and b) what you want your students having access to. Anything I left at the school I would consider loaning to a student overnight to study, though YMMV.
MA libraries come in two flavors: style-specific and philosophy/history. Style-specific books would be the ITF encyclopedias by Gen. Choi, or Kacey's YCTA books. Mine, if I could afford them, would be:
Korean Karate, by Son Duk Song.
Black Belt Korean Karate, by Son Duk Song.
Tang Soo Do Vol I, by Hwang Kee.
Tang Soo Do Vol II, by Hwang Kee.
The Kukkiwon had a decent CD encyclopedia out several years ago that looked like it would be a good choice for a Kukki-TKD student, and of course Gen. Choi's encyclopedia for ITF lineage.
Philosophy/history books are where libraries of different styles and lineages can really come together. I would recommend Rick Clark's 75 Down Blocks to anyone. Most karate-descended schools would do well to have a copy of Funakoshi's books around: Karate-do Kyohan, and its earlier version, Karate-jutsu. Miyamoto Musashi's Book of Five Rings is not only a great strategy treatise, I found it to be a particularly interesting read - but maybe that was just the translation.
I'd also recommend Bruce Grant's Guide to Hangul and Hanja to students of Korean arts, if they are at all interested in the language.