Pratchett books are veritably stuffed with in-jokes - that’s half the fun of reading them. I particularly liked Soul Music in which one of the characters, a rock musician who’s a dwarf(i think, it’s been a while), is often asked… “Aren’t you a little… elvish?”. Hee hee.
Fran
In Haroun and the Sea of Stories by Salman Rushdie, Iff the Water Genie complains about an outbreak of “talking helicopter stories” – presumably a reference to the Duchess of York’s attempt at children’s literature.
Clive Cussler writes himself into most of his novels. Dirk Pitt, Cussler’s fictional hero, always a sense of deja vu when he meets him.
Francesca, Soul Music is basically a music reference fest. That said, the best joke along those lines is that the main characters name is Imp y Celeon(sp), which translates to Bud y Holly.
For Pratchett injokes, go here