I've been running a solo Advanced D&D 1st ed. game for a couple of years now. I just use the core three books (Monster Manual, Players Handbook, and Dungeon Madters Guide). I make everything else up! I've developed a lot of custom rules to facilitate solo play, but hew as closely to the rules as written as I can. I play for 2-4 hours once a week. It keeps my gaming juices primed! My campaign setting draws heavily from H. P. Lovecraft, Clark Ashton Smith, and Jack Vance.