I'd rather see a book turned into a movie. There are a great many excellent books that could be made into really good movies. Good examples of books already made into movies are Lord of the Rings, Jaws, Jurassic Park, Ready Player One, The Road, The Shawshank Redemption, Forrest Gump, The Princess Bride... just to name a few. Seeing the imagery in a book brought to film can create something amazing. And in some cases the movie is better than the book.

Movie being turned in to books, or Novelisation as they used to be called are invariably poor.
Allen Dean Foster and James Blish used to be the go to guys for this, having written novelisation of Poltergeist 2, all of the star Wars original trilogy, Star trek, Alien and the Entity, back in the seventies and eighties.
(To be fair Foster did write some unofficial Star wars sequels that were not half bad)
However more recently in the 1990s Vonda McIntyre has written some good adaptations of films in the sci-fi genre usually putting back in cut scenes from films more character development and often improving bad dialogue.
But in this day of extended editions, director's cuts and deleted scene extras I think the day of the novelisation is over.

I'd love a book about a book that became a moviebook.

Much prefer the book first, and to have read it, otherwise I have the film's imagery in my head

Neither...I usually like the book more...I am just odd!