Here's a question for all the authors: Have you ever been invited to join a book series or an anthology where you ultimately turned it down for one reason or another? I'm thinking either a "Thanks, but I don't really want to write about X" or something where the scheduling/deadline wasn't ideal.
And if you have turned an invite like that down, how tough was that decision? Did you regret it later on?
Of course, but, honestly, this falls into the category of "good problems to have." If you have to turn down work because you're too booked up, or have better options, you can't really complain.
But, yes, I had to pass on at least one juicy movie novelization because it was on a crash schedule (as movie novelizations usually are) and I was too booked up to squeeze it in. Sometimes you can talk to your various editors and shift deadlines around, but that's not always possible.
And sometimes, if you're lucky, you simply have to choose between competing projects.
True story: the reason I only wrote one ROSWELL novel, after writing the first novel based on the OG tv series, was that John Ordover offered me a three-book deal to write THE Q CONTINUUM trilogy at the same time. I would've been happy to write more ROSWELL books, and had already submitted a few possible plotlines, but the STAR TREK deal was too good to pass up, for various reasons, and I couldn't write an entire Trek trilogy and more ROSWELL books at the same time, so I had to ditch ROSWELL for Q.
No regrets there.
Oh, one more: I was once approached to write a kid's picture book about skateboarding. What the heck? I have never skateboarded in my life so I was absolutely the wrong person for that job, which I quickly turned down.
To be fair, I had previously written a kid's picture book about Bigfoot for the same publisher, which was presumably why I was on their radar. "Hey, Greg did a good job with that Bigfoot book. Wonder if he'd be interested in the skateboarding gig?"