Tie-in book lines always start with the publisher, not the writer. In order for a tie-in book to happen, first a publisher has to buy the rights to do books based on that property. Then they reach out to authors -- or, sometimes, authors reach out to them. But the process for tie-ins is almost always the same: the author has to write an outline that is approved by the copyright-holder before a single word of the novel is written.
I don't know about the Reynolds Who book, but in the case of Jim Butcher, that came about because Simon & Schuster got the rights to do Marvel novels in 2004, and one of the editors handling the line was Jennifer Heddle, who was the editor who pulled Jim's first Harry Dresden book off the slush pile at Roc Books and said "we should publish this." In the interim, she moved over to S&S, and she knew Jim was a huge-ass Spidey fan and said, "Hey, you wanna write a Spidey novel?"
Also, apologies to @hbquikcomjamesl for misunderstanding his on-spec comment.
I don't know about the Reynolds Who book, but in the case of Jim Butcher, that came about because Simon & Schuster got the rights to do Marvel novels in 2004, and one of the editors handling the line was Jennifer Heddle, who was the editor who pulled Jim's first Harry Dresden book off the slush pile at Roc Books and said "we should publish this." In the interim, she moved over to S&S, and she knew Jim was a huge-ass Spidey fan and said, "Hey, you wanna write a Spidey novel?"
Also, apologies to @hbquikcomjamesl for misunderstanding his on-spec comment.