Here's my impressions of the Shatnerverse books that I've read:
-The Return was pretty interesting in how it handled the Borg and Starfleet's efforts to combat them. It also gave the Romulans something interesting to do. A bit wanky in terms of Kirk beating the crap out of people, but he was flooded with Borg nanoprobes at the time, so it isn't like he suddenly became an uber badass with no explanation.
-Avenger was pretty freaking forgettable. I read it once and I don't remember anything about it besides the plot revolved around a disease.
-Spectre is pretty good, balancing out the TNG and Kirk plots fairly well. A pretty interesting take on the Mirror Universe and a decent cliffhanger ending.
-Dark Victory is really "meh." The beginning and end, which involve the Mirror Universe, are interesting, but the middle part about the Starfleet conspiracy and Kirk's weird ass kid is not really interesting.
-Preserver is the last one I read, and the beginning is the most interesting part, because it involves the Mirror Universe and the First Federation. It's kinda weird though, because it starts in media res and doesn't really bridge the gap between the end of Dark Victory and the beginning of Preserver. Some of the stuff relating to the Preservers is kinda interesting, but it doesn't really mesh and the ending is kinda incomprehensible.