Tomb of Horrors, a place filled with death traps and powerful monsters where even the fake versions of the demi lich Acererak could take down several adventurers. I managed to find a version updated for DnD 3.5, which, while very severe, is actually a toned down version of the original, acording to the Tomb of Horrors TvTropes page.
Although the chances are that you'll lose half a dozen characters before they get into the tomb, this Nintendo hard scenario was voted the 3rd best ever in Dungeon Magazine 116.
Something clicked in my brain after reading this. I've long been an advocate of having decisions in gamebooks having logical consequences and making sure that characters are not killed early and often for making a wrong turn or for not picking up item x three rooms back.
The evidence has been around me for a while; I just hadn't noticed it. For example, there are several RPG systems such as Call of Cthulu or Warhammer Fantasy Roleplay where death is not only inevitable, it is usually favourable to the alternatives (going insane from seeing an eldritch abomination or being mutated by the forces of Chaos for example) or it is just an accepted part of character creation such as in Dungeon Crawl Classics. Despite the inevitable bloodbath, people are still entertained by these games and continue to play them.
This Grognardia retrospective on the Tomb of Horrors provided a little insight into why it is still entertaining - it appeals to gamers as a challenge in order to win. Being able to win the Tomb of Horrors would certainly be an achievement by showing off your initiative and problem solving abilities.
Adventurer blog post, gamebooks cannot approach such challanges in the same way as the gamebook author cannot anticipate and plan for every option available to the players in advance. However, there is this great post from Fighting Fantasist which quotes Pete Tamlyn in this article stating that if all of the decisions in a gamebook were fair and had logical consequences, then the gamebook would be far far too easy (on the other side of the coin, some Lone Wolf books are too easy because of the logical consequences to decisions but they still provide entertainment due to Magnamund being such a rich fantasy world to explore and the fact that Lone Wolf is an epic saga).
Which leads me to the reason why gamebooks such as the Warlock of Firetop Mountain and Deathtrap Dungeon are considered classics despite the fact that they are full of decisions with arbitrary consequences and victory being decided on whether you just happened to have picked up the correct items for the end.
They are challenging and some people like a challenge (such as the champion and puzzle solver gamebook player type). Even an unfair challenge.
Also, all things considered, the penalty for losing these games is the death of an imaginary character who you rolled up three hours ago. Not big when you think about it. It's not like it was a level 17 rogue/ranger who I had put months of effort into.
I guess when I was younger, I got too precious about my characters dying and wanted to cheat to save them, but, by reading about the Tomb of Horrors, RPGs where characters always get killed off and blogs of gamebook playthroughs where most of them end in failure (here, here, here, here, here and here), I have come to the conclusion that, with certain provisios such as fair dice rolling and being part of a fun story, having your character die horribly or fail to get the treasure doesn't necessarily put a dampener on the entertainment value of a gamebook.