Another novel with an interesting "spin" on Early Trek is
The Children of Kings.
While at first I'd thought of this as "just" a TOS novel with other names, the more I (re)read it, I saw a darker and to me a more realistic side of Trek.
I know it's not strictly in Continuity (and IIRC that was a deliberate effort by the author), but I would have liked to revisit this version of Pike's Enterprise.