Having read most of the books in this series, I have yet to find one more difficult than Crypt of the Sorceror. There is only one successful path through the adventure (yielding all the items needed for the final battle), and it requires beating several extremely strong foes (a SK 12 ST 24 Pit Fiend for instance - with stats equivalent to the highest a player could possibly roll) and making several lucky rolls (such as avoiding a skill penalty in the first encounter!). The book is thus effectively impossible to beat "fairly" (without "cheating" at roles, fights or stats).
This aside - finding the right path is an intriguing challenge, with a lot of different path options at various points. The detail of the fantasy scenario is also very effectively constructed; the reader is drawn into the plains, pits and dungeons of the adventure, and there are some unusual monsters along the way too. Very well written, but I'd have preferred if it were somewhat less frustrating to play.