Do publishers send books for review, or are the books bought independently?
Short answerBoth, with some books also loaned, but only recommended works appear on the channel because negative video reviews would not give authors a comparable right of reply.
NotesBooks come through purchases, review copies, and loans. Only works considered worth recommending receive public discussion. The reason is procedural fairness: a journal review normally gives an author a recognized venue to reply, whereas a YouTube criticism can reach a large audience without offering the same remedy. The positive tone of the book segments is therefore a selection policy rather than evidence that every book read receives a favorable judgment.