I personally would never buy a UE4 print book until the engine (or feature set) stabilizes. I would consider buying a digital book that would be updated for the next X months-1 year. A digital book would also get around the hesitant publisher problem mentioned, since you can get anything published digitally. (Possibly NSFW)](Q&A: The Women Who Write Dinosaur Erotica)
I would also be less inclined to buy it if it wasn’t written by someone who has worked with any UE in the past. That’s just my personal (and prejudiced) preference. Those people know best, “the ‘unreal’ way” to do something. People who worked under deadlines and had to ship a real product would have great experience and ‘war stories’ to share. Games engines that are this big and advanced usually have quirks and pitfalls.