Make novels be wishable in normal and explore modes in addition to
wizard mode. I don't think this weakens the tribute and it prevents
someone who attempts such a wish from getting misleading feedback of
"Nothing fitting that description exists in the game."
Wishing for "novel" will yield "novel named Foo" where "Foo" is a
randomly chosen Discworld title. Wishing for "novel named Bar" will
yield "novel named Bar" or "novel named The Bar" if "Bar" or "The Bar"
is a valid Discworld title, or else override "Bar" and pick random
Discworld "novel named Foo" if it isn't.
Since first read of a novel bestows some experience (once per game, no
matter how many novels become available), a pacifist with an early
wish can get a head start. I don't think that's a big deal. And it
will require an awful lot of wishes for any player who wants to acquire
all 41 titles in one game. I imagine someone will manage it.