Who ever thought a Fantasy Gamebook could be both exciting and hilarious?  These are without a doubt my favorite gamebooks, and I only wish J.H. Brennan could have written more.  Where else (other than Monty Python) could one fight a Vorpal Bunny, build a real origami boat to escape a deserted island and fight a walking Zit?  In fact a lot of the humor was Monty Python-ish which is not surprising since the author was British.  I still dread turning to page 14 to this day... (for those of you who never read these, page 14 was always the page you turned to after getting killed.  Here you'd get resurrected and start over).

Demonspawn was by the same author but surprisingly lacked the humor of the GrailQuest series.  I also remember them being very difficult, with very tough battles.  As far as gamebooks go, they were good quality though.

Monster Horrorshow was an attempt to turn the game system into a complete role-playing pen and paper system, but this was the only such book released to my knowledge, so I don't think it went very far.