I think that must be an Amazon data error – I have the first three volumes and they’re about 230 pages each, so pretty short by LN standards. (My volume 1 I picked up in a bookshop on a whim and it’s signed by the author…) I guess on balance I’d recommend it if the back cover blurb/series premise sounds interesting (I did read the first three books in the series) but it’s not a book I’m “you need to read this right now” enthusiastic about.
A bookshop-book I can recommend more full-heartedly is the ビブリア古書堂の事件手帖 series, which I liked a lot – interesting individual stories centring around different books or authors, and an overall series arc which knows where it’s going and doesn’t outstay its welcome.