I've only done 3 Fighting Fantasy books, Zipp, so that Citadel of Chaos deal is unfamiliar to me. What's the issue with that monster or whatever you mentioned?
My three were "Trial of Champions", which I've described my feelings about.
The "Snow Witch" one, which was more lenient, although still brutal, was kind of neat. You kill the Snow Witch about 2/3 of the way through it, but you and one of two prisoners you rescued are cursed by a slow-acting poison-curse deal, which leads to you having to get cured before the gradual loss of HP kills you.
A Samurai one I have kicked ass. You also had "Honor" as a stat and if you acted in a way that stripped you of all your honor, you immediately "game over'd". It was really neat in that you had two plot branches. Each one potentially gave you 3/6 of the book's summons. Late in the book, you fight a demon who summons three monsters. You have to guess what summon to use on what monster. You get all three right and you only have to fight the demon. BUT, you only auto-lose the book if you didn't get any of the three summons from your path (you get overwhelmed with the battles).
Also have about 3 Lone Wolf and 3 spin-off Grey Wolf books. Lone Wolf is a series that you either do ALL the books or you cheat, as I learned because the first book I played was the third. You get the Sommersword in the second. In the third (and future books), there are a number of times where you are disadvantaged (or simply fucked) if you don't have it. And of course, being able to add new powers for each book you do is HUGE. That's what makes Project Aon so awesome...I can do the books like they were meant to be done, instead of pretending I had all the stuff you're potentially able to obtain throughout earlier books.
Along those lines...Zipp, what is the bizarre deal with Cauldron of Fear and how the Sommersword is devastating to you for the final battle? If you have it, the dude is 44CS/50life. If you don't have it, he's 34/40...and you get the ability to potentially one-hit him if you have a magic dagger. It goes against the entire grain of the series (that the Sommersword is this holy weapon of goodness) when possessing it makes a villain near-impossible to beat (as opposed to simply very tough...or an auto-win).
One I really liked was the GrailQuest series. Entertaining writing and a neat vibe, where you get TONS of items that give you huge battle advantages over most foes, but there is a HUGE emphasis on puzzle-solving and being thorough (which is marred a bit by a couple cheap auto-deaths just for going everywhere you can). You have the same mandatory-fetch-questing that the most cruel FF games have, but you can repeatedly visit places if you screw up.
I'm not afraid to die because I am invincible
Viva la muerte, that's my goddamn principle