I have just read "Book Girl and the Captive Fool" and even though it wasn´t as good as the second book, which is so far the best of the series imo, it was worth the read.
Spoiler for Captive Fool:
The Third book was kinda like a prehistory of the 4th book/movie (people who saw the movie will eventually understand me
). It was imo just to befriend Inoue with Akutagawa. The psychological aspects weren´t as good as in the other books. But I liked the Epilogue, which was very interessting and thrilling. Also the parts with the "heartless" Takeda were really interessing.