there's a series of in-universe biographies that ranges from good to great. The Autobiography of Mr. Spock and The Autobiography of Benjamin Sisko are my favorites, they're very good looks into those character's minds and also give you a lot of cool background info on the federation inside and outside of starfleet. i definitely recommend those two and the rest of you're hungry for more after

if you're salty about how Enterprise ended, espwcially for one character in particular, The Good That Men Do is worth checking out. it reads like a fixfic because it essentially is but hey. it leads into a few other books but i haven't had a chance to read them yet