Well there was one for Revenge of the Sith in 2005, so it wouldn't be that implausible. It's a real shame if Disney is putting the kibosh on this for political reasons and not wanting to "rock the boat". I just want an in-depth account of the production, warts and all.
It's not political. It's business. There's no business case to be made for releasing the book, particularly the "Warts and all" version. At best, it'd be a mildly interesting curio for hardcore fans. At worst, it'd taint the brand, **** off the creator, and expose some of the more fraught aspects of getting the new series off the ground that maybe Disney doesn't want seeing the light of day for a while (if at all). None of which is good for long-term sales.
That's the thing to remember: Star Wars is now an ONGOING film franchise with no end planned. It's like the James Bond series now, only with multiple spinoffs and such. Disney's plan is to make it into a cinematic juggernaut.
Prior to the handover and TFA's release, Star Wars was, mostly, a dead letter, and a niche product at best. Sure, it's a beloved film franchise, but the only people buying the stuff were kids buying Clone Wars stuff, and hardcore fans (a minority market) buying whatever struck their fancy. Everyone who wanted the movies already had 'em in some form or other, and the PT had split the community.
In that set of circumstances, there's no real reason NOT to release a "warts and all" book. Particularly if it's more a discussion of just the process, without casting aspersions on the creators. At worst, you'd hear about Gary and Marcia's exit from the franchise early on, but that's all ancient history.
With TFA, it's a different story. We know Lucas had a whole other idea for where the franchise should go. We know Disney told him "no thanks." That's as gossipy a story as we're gonna get on that one, because there's just no reason to crap on Lucas at this point. He's out. He's not involved in the franchise at all anymore.
They're no longer his stories. BUT, to discuss how the film got made would necessarily expose the discussion where his ideas were rejected.
And here's another consideration:
The story itself is ongoing. A big "behind the scenes" book, or at least one worth its cover price, would discuss all manner of things that are likely chock full of spoilers. The direction the series is headed, the nature of the Knights of Ren, Snoke's origin/identity, Rey's parentage. Any of Finn's backstory other than his stormtrooper memories, etc. It's tough to do a "behind the scenes" book without touching on that stuff. We might get one after this trilogy wraps, but even then, maybe not depending on the future of the franchise.
Again, it's all because it's an ONGOING concern now, not just "Some movies we made that are done with now."