I'm not 100% sure, but I believe "Sepher Sephiroth" is supposed to mean "The Book of Numbers," or something about how one ascends towards Godlikeness, in Hebrew. (Incidentally, "Safer Sephiroth," the name of the final boss, is a mistranslation of "Sepher Sephiroth").

I know very little about this myself, however; I'm basically remembering rather poorly something Kishi posted a long time ago. At any rate, I'd recommend searching for "Sepher Sephriroth" on Google, not just Sephiroth :P