"How to build a car", by Newey is well worth a read.
It's amazing how much he has achieved; beng one of the last people who has an overview of every aspect of the car, rather than engineers today that due to the vastness of the subject (and through no fault of their own), can only specialise in certain aspects.
Joining Ferrari may not be a big leap for him, but it would certainly keep RedBull lawyers occupied separating out the 'intelectual property'.
It may just intended to be a wake-up call to RedBull, to get their act together?
...and the book was available on Prime Reading for a while. I can't see it there now, but it may reappear at some point?