Book 2 of the Legacies trilogy, Best Defense, picks up right where Book 1, Captain to Captain, left off... plot-wise, at least. The quality picks up in an entirely different way. This is to be expected, though, because the author is David Mack, who is a superior Trek author.
Book 1 ended with an exciting cliffhanger involving a Tal Shiar agent having stolen the Transfer Key -- bad, bad news for the whole universe! What's more, it quickly becomes apparent that secret agents, probably hired by the Romulan Empire (or at least the Tal Shiar...) are trying to sabotage the Organian peace talks that would establish some kind of agreement between the Federation and the Klingon Empire. The plot revolves around these conflicts, as well as keeping track of Una's progress in the alternate universe (which there really isn't much of).
Mack is a skillful writer, so that not only do the classic characters' "voices" sound right (especially Kirk's), but the descriptions of the action are evocative and compulsive. One can "see" what's going on easily, and it's always apparent what the stakes are in any given scene. Admittedly, the stakes are raised on a personal level for Spock and McCoy in a manner that seems a bit contrived (i.e., artificially and implausibly constructed for the sake of advancing the plot), but Mack propels the action so smoothly that I stopped caring about the contrived nature of a couple of the plot devices. Indeed, the final confrontations on and around the planet Centaurus are so nail-biting in their intensity that I didn't mind or even notice any literary shortcomings at the moment of reading.
And now the action moves into the final book of the trilogy -- Purgatory's Key, by Dayton Ward and Kevin Dilmore. Too bad it's not David Mack again, because he is reliably entertaining. I hope Ward & Dilmore can live up to the overall quality of the first two books, because the story is interesting and the stakes are high... these things demand good writing.