Despite the very prominent depiction of Spock on the cover, there's little of him physically in this novel - so don't be deceived by appearances.
Rather, Cast No Shadow is on the one hand a quite fast espionage-thriller, featuring Elias Vaughn on his first... well... assignment as a field operative for Starfleet Intelligence is too strong a word since he shouldered himself more or less into this mission, and on the other hand Valeris' journey to reconciliation with who she is and what she's done. Being that the book deals with events between the TOS-movies and TNG, I'd rather have seen it labelled under the "Lost Era"-subtitle.
The plot starts 7 years after Star Trek: VI with a devastating explosion near a starbase where relief-deliveries are transferred from Federation ships to Klingon vessels. Klingons are quick to blame a renegade clan within the Empire but there is circumstantial evidence that points in another direction - a group called the Thorn, basically a bunch of freedom fighters from a world called Krios (TNG's "The Mind's Eye", "The Perfect Mate") which was annected and stripmined by the Klingons. Which in turn leads to Valeris because the Thorn were involved in the assassination of Gorkon. Starfleet dispatches a team to work with the Klingons to solve the mystery - but not everyone is willing to listen.
Besides the obvious motivation to kill as many Tyrants (Kriosian for Klingons), they also try to put a wedge between the Empire and the Federation - aiming not only for Klingons but also for Starfleet relief vessels. The rather fragile Khitomer Accords are even more put in danger by power-hungry generals and (to a lesser degree) officers who aren't willing to follow up on leads. Vaughn, believing himself ready for something greater than just data-analysis, joins seasoned Starfleet Intelligence officer Darius Miller in a mission to discover what really happened - and finds himself in command of said mission (including juggling a Klingon intelligence officer and the traitor Valeris, not to speak of a very short timeframe to find the next bomb, the next target and to prevent the unspeakable from happening) when Miller gets killed. Jack Ryan, any one? This part of the novel is quite straight-forward as Vaughn learns that there is no mere black and white, especially in intelligence operations, but many shades of grey in between.
The conspiracy against Gorkon certainly gets a bit fleshed out as well - a bit of Chang's background (including Shakespeare quotes), Cartwright's background and eventual fate, even the Romulan ambassador finally gets what he's got coming. I also enjoyed the rather short appearance by Endeavour and Sulu - I'm not too fond of that character but from what I've been reading especially in the Lost Era now his crew could be quite interesting.
But the part I've been really looking forward to (Klingon internal politics have become stale and repetitive, after all - and of course, at the end, what truely happened is covered up) was Valeris. In TUC she came across as aloof, determined and cooly logical - and honestly, I could see a Vulcan arrive at the decision that an alliance with Klingons is to the Federation's detriment. After all, by the beginning of TUC the Empire was on the verge of total collapse, so the question of why help them, why not let them die, why expend the ressources, why risk opening yourself up, is valid. Unfortunately, though, this novel goes down another road. It uses the childhood trauma-device, coupled with nice father-issues, combines the emotions these 2 elicit with Vulcan repression - and gets deep-seated unrealized hatred and prejudice against Klingons as a species as a result. Not that this makes for a bad story, not at all, but it sort of diminishes her superiority, her cool arrogance in TUC. I guess I'd expect more from a Vulcan than those "common" underlying issues.
Pondering this I wonder why her dissatisfaction was aimed solely at the Klingons - and not at her father and the Federation in general that he represented. After all, while the violence with which she was confronted might have been horrific, what really did her in were her father's actions... putting a treaty before his family, deciding to negotiate from a position of apparent weakness (by not going in in a heavily fortified ship in the first place).
On the other hand, she gets played by every side, I guess. Cartwright knew what happened on that planet when she was a child - and exploited that ruthlessly. The question remains whether that incident is public knowledge or if it was covered up to avoid a larger-scale confrontation with the Klingons back then. Tancreda took note of the fact that every male Valeris cast in a paternal role took the side of "the greater good" instead of supporting or even listening to her... and tried to influence Valeris in that direction in her sessions at the penal colony. Given that Tancreda (along with Cartwright - see Perry's "Cloak") also belongs to Section 31, who knows what her purpose in doing so was? Perhaps positioning Valeris in a mission against Spock whose politics oppose Section 31's?
What remains is a deeply troubled character. I am not sure whether her nonchalance at the end stems from her still not regretting anything or from her being still stunned at the realization of the emotional roots for her deeds that the mindmeld with Rein uncovered (which I'm leaning towards). I'm happy that Swallow deliberately left this unresolved so that the reader can choose for themselves and, in this case, didn't take the easy way out in redeeming her by a heroic and/or self-sacrificing act. There's no black and white, again.
One of the most disturbing scenes in TUC is the forced mindmeld Spock uses on Valeris. And while Cast No Shadow doesn't diminish the brutality it emphasizes its use as a last resort and the fact that a mindmeld, even an unwilling one, is a two-way street. Although Spock (and in this novel Valeris) is the instigator of the meld, he is not untouched by it. It was necessary, back then and here, but not without consequences with which everyone involved has to deal... some better than others. It's a fine line between excuse and explanation of the act that Swallow draws here. One I can live with.
One throwaway-line worth mentioning alludes to the destruction of the Enterprise-A in one of Shatner's novels. Nice nod to continuity here.
There are quite a few plotthreads that could be followed up on in a sequel. By my account at the end of the novel, Valeris is just in her mid-forties. So who knows what the future will bring?
Cast No Shadow, in any case, had an exceptional premise with its first very exciting 100 or so pages, then it tapered off a bit. I was sad to see Miller go but it was necessary to throw Vaughn into the deep end - being a mentor certainly doesn't bode well for a character... And, of course, the IMO oversimplification of Valeris' motivations did not quite meet my expectations. In the end, what remains is a fun ride, sprinkled with Klingon politics, conspiracies and a bit of a glimpse behind the facade. Entertaining - but not outstanding. 7/10