Not a book but a blog: A collection of unmitigated pedantry (aka acoup.blog), which a few days ago Had an article on the Marian reforms or how they weren't actually a thing. Specifically: Lots of military reforms that commonly get attributed to Marius were either already in place before Marius, or appeared long after Marius. They just all kinda get attributed to him because of interesting historiography reasons.
Definitely an interesting read in no small part because it directly contradicts the Total War standard.