I tried to enter the letterbox URL but apparently, if a film doesn’t have a Wikipedia page, it can’t be included.

Will look into this, if you remember/stored the ones that didn't get included, you can log them manually by using the search function to the right, if it says the same thing even though you try to include tmdb/letterboxd link there, please let me know!

Does it work the same with books? What would the parameter be?

Not yet, we need to find a good API that's cheap/free, currently we only rely on Wikipedia.