It's difficult to keep up with a continuing series when the books come out a year apart. "Vanguard: Reap the Whirlwind" (published May 2007) is the third book in a series that started with "Vanguard: Harbinger" (published July 2005) and continued with "Vanguard: Summon the Thunderer" (published June 2006).
In the case of ordinary Star Trek books this perhaps wouldn't matter quite so much, everyone knows who Kirk, Spock and McCoy are, or Picard, Ricard and Data. But this series brings in a completely new cast of characters. Set in the period of The Original Series, it is placed on a space station in the distant Taurus Reach, an unexplored area contended for by the Federation, Tholians, and Klingons. It also contains a deep secret in the form of a massive genome left behind by a dormant, extremely ancient, and massively powerful people.
That said, there are a few characters from TOS, and in one case what might be a section of suspense isn't because anyone who has seen "The Wrath of Khan" knows a particular character is not about to die. But there's also a delightful surprise at the end when another character from that film suddenly appears.
Picking up "Reap the Whirlwind" a year after reading its predecessor, it takes a while to get up to speed. Author David Mack does a good job of reminding us who everyone is, and there is a very helpful "Vanguard Minipedia" in the back of the book, which unfortunately many readers may not discover until they get to the end of the book.
The story is good, many threads come together, and the book brings up interesting issues of ethics (or personal feelings) versus duty, or whether a potential threat to the survival of billions is worth a much more certain threat to thousands.