I noticed the same thing last night. It's still in the spell file (verified with a parser), but has disappeared from my book. I *KNOW* I've cast that spell quite a bit previously.
I also *think* (not nearly as positive on these) that one or two other spells (like the vision spell) disappeared as well. And strangely enough, I don't remember buying (or even seeing) "harvest", but that is now in my spellbook. I'm wondering if spell IDs somehow got rearranged.