If you’re looking for any type of book (free or not) you should definitely check out this guy’s YouTube channel.
Not too long ago he posted this video that he talks about what are maybe the best options you have if you’re learning Japanese.
He shows some really good kanji books, books to prepare for the exams and grammar books that you’ve probably never heard before.
He also puts in the description a lot of important pdf files you can download for free if you can’t buy those books.
Keep in mind that most, if not all, of these books are available on the Japanese Amazon and I don’t think you can find them anywhere else. Also, I’m not promoting his channel or anything, I found about him on my own a couple of months ago and I believe I should share his content with anyone that’s studying the language right now.
Here’s the video :