This is a bit of a longshot, but I've posted the same info to other boards I've been on over the years, so why not here as well?
There are two books I remember reading but have never been able to to track down.
The first was a young-adult post-apocalyptic novel about a future Earth where a nuclear war has knocked society back to a medieval level, and anyone who knows anything about science is regarded as some kind of mage. Humanity is threatened by mutated superintelligent rat creatures. The story follows a mage and his apprentice who are looking for an ancient treasure that turns out to be a computerised encyclopedia. About the only other thing I can remember is that metal is really rare - a character has a small metal dagger and it's regarded as almost magical.
The other is a sort of horror novel where the protagonist is forced to go on a kind of treasure hunt through modern day London at the behest of a bunch of bored young aristocrats. If he wins, they bring him into their group and make him rich, if he loses, they kill him. He has to solve a bunch of historical puzzles and isn't allowed to ask for help from anyone - at one point he gets help from a homeless man who the aristocrats kill. In the end he wins and then turns into a soulless monster just like the rest of them.
Does anyone have the faintest idea what books I might be talking about?