Though not as ambitious or compelling as The Return - With Avenger, William Shatner (with the help of Judith and Garfield Reeves Stevens), once again proved he is a first rate Star Trek author.
In his previous novel, he wrote an extremely ambitious story and dared to write a story challenging some of Star Trek lore. However, it was done in such a way that you were forced to keep read. While, The Return lacked a solid plot - it was just a long drown out action scene - Avenger however, is just as good - maybe a step slower in the action, but has a plot that requires more thinking. In fact, most of the action came in the end.
Plot-wise, the novel sounds exciting... Kirk and Spock together again, joining forces to save a "new generation" from "an awesome menace unleashed by a ruthless interplanetary conspiracy". They are chased by hidden assassins and, run into old lovers, unlikely new allies, and eventually are confronted by the one man who might be able to stop them: Captain Jean-Luc Picard of the Enterprise-E.
Like The Return, Avenger was slow to start... but you better pay attention, or you are going to miss the point of the entire ending. While, it may help to read the first book of the trilogy by Shatner, Ashes of Eden - I did not and I was okay - there are a couple of overlapping characters here.
Interestingly, in the last two books that Shatner co-wrote - Kirk and Picard seem to always be at odds. Admittedly, both novels were excellent, so apparently this formula works. A nice surprise is when Kirk runs into the Emergency Medical Hologram (EMH) - that was truly a classic moment. Do not be surprised when a very old Dr. McCoy shows up.
Overall, I would caution that you should not be biased because "Captain Kirk" is writing this book - although he is very biased himself - because after reading the ending, you will be very satisfied. Just get through the somewhat sluggish beginning and you will be in for a treat!