But the categorie "A book made into a movie you've already seen" excludes me, I think, because I always read the book before see the movie !
There are some movies I've seen that I hadn't known were based on books until long afterwards; you might be able to find one of those. Or look for a new film based on a book you haven't read yet and break your "book first" rule on its account!
Of the obscure-books-turned-into-popular-films I'd include The Bishop's Wife - the 1940s Christmas movie starring Cary Grant; the novel was rather darker, and I admit I didn't love it, but it's one of many examples of didn't-know-there-was-a-book.
and it sounds interesting. I also like how the challenges are varied. However, 40 books feel like a lot. I am now doing the 666 challenge and the Book Riot Read Harder, and to do another one is just too much and leave me little time to read something that I just want to read.
Maybe I will do this instead of Book Riot, as frankly I am fed up with its obsession with "people of color" and gender orientation. I mean, I enjoy reading all types of book, but Popsugar's "A book by an author of a different ethnicity than you" sounds much nicer. "People of color" basically means there's white and then there's the rest of the world.
As for "A book made into a movie you've already seen" , my thought goes to books like The Jungle Book, Dracula, and many of the older classics. Or just try holding on to a book till you've seen the movie.
The great thing about this challenge is that you can customize it any way you want to. If 40 is too many books for you, then only pick however many books you feel is enough for you. You could always read a book and see if it fulfills a prompt in the PopSugar, BookRiot, 666 or any other challenge.