The tie-in novel is literature involving the characters of a TV series, film or other work and usually written by some author you've never heard of. Pretty much anything can have a tie-in novel. They're often compared to authorized Fanfic, in that they're usually not part of the main continuity, do things that would never be considered in the original, and are widely variant in quality. If a series is exceptionally popular, the tie-novels may have their own micro-continuity. If it's mega-popular, expect them to occupy their own Expanded Universe. Expect tie-in novels by the same author to refer to events in their other novels.