I find that a bit facetious, however, because (IME) it's very, very hard to keep that stuff from creeping into your games. Sometimes it's a whole book, sometimes it's only a feat, sometimes it's a couple spells. And then, before you know it, your "core" is 32 books and 17 Dragon articles.
A few random spells and feats from various books is easily handled by collecting them into either a Word or pdf document.