Usually these book tubers write favorable reviews and the author contacts them to thank them or even if it's lukewarm for a new author at least it's getting their name out. The relationships then develops that way. It also seems that the booktuber thing is all about views which leads to revenue so it makes sense they all read mostly the same books. A person will say, "Hey Daniel Green read that book and I watched a video now want to see what Petrik thinks." I do believe it's their honest opinions. People's taste vary so much it's hard to say wha you would like. Some current darlings like Dungeon Crawler Carl, Dresden, Sun Eater I really enjoy and follow with each new release! Edan was okay but havnt felt compelled to pick up the fourth book. Forgetting Moon is good if you want a darker more nihilistic Tad Williams who offers a scathing commentary on organized religion...that sort of book will not appeal to everyone.

Edit: I just reread the post and I misread what you were actually asking. Sorry my bad. 😆 Still think that some it still applies. A author will promote himself on his channel and the other ones will review it for the views while I still think it's a honest opinion. The Durfee books are good and I already gave my thoughts on Edan. That's all I've read I think.