They're basically solo campaigns with the book acting as a DM, as I see it. However, you could argue that they're not, I guess.
Fundamentally, we're a site about real RPG questions for real situations. I suspect many contrived questions will lack standing or be more appropriate for sci-fi, but if you find yourself using them in an RPG context, they're certainly allowable.
While there is an element of sociality to RPGs, we have addressed solitare in the past. Therefore, if there exists a real problem with the mechanics of one of these, go for it.
One thing to consider is that some solo gamebook series' have an associated RPG (I can name Fighting Fantasy and Lone Wolf), which then brings campaign setting questions into consideration, specifically those related to gamebook events that could affect the multiplayer RPG setting. I think there's enough of a grey area that individual questions should be examined on their own merits rather than having a solid rule against them.