My Enemy, My Ally

I will fangirl these books until the end of time, especially because it's Diane Duane, but you know what? I think these books are good enough to be appreciated by non-Trek fans as well. The worldbuilding is fantastic - she even comes up with a functional language! - and the characters are excellent, strong yet flawed and believable too. I still haven't read the fifth and last book, but I look forward to it if it's as good as these four.