Thanks very much for the rec! Will have to add them to my TBR list.

I'm interested in all the permutations of derivative fiction; from fanfic to authorized tie-in novels to stories like this based on public domain works. Have been trying to decide whether a valid generalization can be made between derivative work which is specifically un-authorized, work for which the original author never had the chance to authorize or not, and work the original author endorses.