I've noticed that about Adele and Ed Sheeran, too. If it's done right, I feel like it could work! But I don't know, it's just speculation right now. I hope that if the artist Adele collaborated with doesn't use their song, she at least mentions who the artist was at some point.
As for the book, I agree completely. I don't know that she necessarily should do it, and I probably wouldn't want her to write an autobiography because, like you said, her music is autobiographical as it is. I feel like for her to write a book about the same things her music is about, it kind of defeats the purpose of her writing autobiographical music. Besides, she'd never do it—she's far too private for that.
Unfortunately, I think you're right about her writing something fictional, too. I wouldn't want her to write any sort of book until much, much later in her career, but I also think that if she wrote something fictional, it would go against the fact that she wants to be a musician rather than a celebrity, which, to be honest, I respect. The only reason I would want a book from her is because I feel like it would be the most entertaining thing ever to read.

She's said in the past that she keeps "drunk diaries." Maybe once she's much older, she could revisit them and write a sort of comedic memoir about her experience in the music industry, kind of like Carrie Fisher did with her last book. I don't know, I really don't want Adele to write and release something that would go against what she stands for like that, but it would be an extremely entertaining read, that's for sure!