1st edition Deities and Demigods, with Cthulu and Melibone. It was part of my brother's collection before he went to college. My brother gave it to me, I think that I sold it to my brother-in-law for some nominal fee, told him that it would be valuable some day. IIRC, he sold it to a friend of his.
Out of curiousity, how much is it worth now?
/me kept his first edition copy of The Silmarillion. Difference being, I'll read that again.
Cheers,
Ben