I'm not sure I agree in the case of The Lord of the Rings, though obviously one should read the book first. Again, sorry to link bomb you, but here are two articles on why, if you're interested. :)

https://medium.com/illumination/the-lord-of-the-rings-book-versus-film-241ff05ba00d

https://fanfare.pub/the-lord-of-the-rings-20-years-on-aafa98effe0e