I don't remember if I ever officially reviewed Harbinger, so thanks for the thread. At the time I first read Vanguard, my overall enjoyment of the 23rd Century and the characters and situations presented in the original Star Trek paled in comparison to my enjoyment of and familiarity with the characters and situations presented in Star Trek: TNG, Star Trek DS9, and Star Trek Voyager. I had seen most of the original ST movies starring Shatner and Co. and knew who the characters were, but had little lingering interest in the Original Series and its setting.
David Mack changed all of that with Harbinger. The novel not only got me interested in the 23rd Century and the situations and setting of the Original Series, it also tured the feel and tone of my all-time favorite Star Trek series, DS9, by creating a world and characters that were recognizable, interesting, and flawed (something that the characters of the Original Series lacked), with standout characters for me being Quinn and T'Prynn (with whom I was already familiar due to her supporting presence in the DS9 Relaunch novel Mission Gamma: Lesser Evil).
I really enjoyed this book, and, with it (as well as with the other Vanguard novels he's written to date), Mr. Mack cemented himself as one of my favorite Trek Lit authors (company he shares with his fellow Vanguard authors Kevin Dilmore and Dayton Ward, DS9 Relaunch author S.D. Perry, and Christopher [L. Bennett]).