There is a long precedent to writing in the world of another author - it's been happening for thousands of years
Although - serial to Hitchhikers? Wasn't poor Adams trying to KILL off the mere IDEA of a sequel by destroying earth, all alternate earths, and all alternate versions of the main characters?
HOWEVER - something I find very irritating is misleading author information, like JAMES PATTERSON'S <whatever> written by a team of unknowns
or ruddy Victoria Andrew's books, written from beyond the grave (and some might argue, beyond the pale). This goes beyond mimicry (acceptable, imo) to an outright intent to deceive.
While I wouldn't tend to write in other authors' worlds - too many ideas of my own - I know I'd be flattered to the point of having
to try it if, say, Piers Anthony asked me to write a Xanth book...