I'm kind of on the fence about how to rate this one. On one hand, it tugs at my various heart strings because it takes place in an alternate reality (kind of - and I love, love, love the potential stories which can ensure in that environment), and it has a tremendous amount of, for lack of a better word, longing, nearly romantic longing, between Kirk and Spock. But, on the other hand, it's a rather mild version of an alternate reality. By which I mean, apparently the universe itself remembers what the timeline is "supposed" to be like, and any major alterations thereof cause the whole thing to fall apart...like a body rejecting an organ and an organ rejecting a body...as alternate-Spock states.
Essentially, the Romulans go back and ensure that the Federation is never formed, but in its place is a region of space controlled by Vulcans and the Human League of Planets, who have joined forces in an Alliance. Meanwhile, the Romulan Empire, which they were trying to make more powerful with the experiment, is only marginally larger. The Alliance uses the same style of ships as the Federation, only the crews are more like 50/50 Humans and Vulcans, with Captain Spock commanding the ShiKhar (the Enterprise equivalent)...and the whole crew are pretty much in their usual places: Scotty is Chief Engineer, McCoy's the Doctor, Sulu, the Navigator, Uhura on Comms, but, Chekov is the First Officer and Kirk is a lowly Ensign (but, he's not a plucky young one...he's a bitter, grizzled, mid-30s washout Ensign). Starfleet still exists, too...although it is only half the size of its Federation counterpart - and is still able to balance against the Romulans. Those poor, poor Romulans...these novels really rake them over the coals over and over and over again. Oh, Captain Pike even makes an appearance as the Captain of another Vulcan "Constitution-class" ship.
However, many people are experiencing nightmares as their brains are trying to "remember" their actual selves, which run counter to their alternate selves, and the more drastic the difference, the more damage is being done to their psyches. Spock even theorizes that the universe has roughly two weeks left before half the population of everyone everywhere goes completely insane. However, with the help of the Romulan Praetor, Kirk and Spock travel back in time, stop the Romulan agents, and fix the timeline - you just need to go Warp 17 and slingshot around a star.
There's a smattering of non-canonical history here, as well, what with Humanity apparently founding the Federation...in 2097...on Earth...before having met any aliens...but, the novels are known for their lore drifting.
As an aside, Captain Spock is suffering from Pon Farr and mates with the Romulan Praetor, a lovely lady named Thea, who is responsible for the time-travelling events and is the only one (well, kind of - more like the entire crew of her personal battlecruiser) who will remember the events in both First History and Second History (as they call the timelines) completely as she was in a protected environment.
There is also a mention that the Romulans have conducted small-scale time travel experiments to try to improve small sections of the Empire, such as altering the history of one planet to make them stronger, only to have it result in that planet's self-annihilation due to a global war. Or when they apparently tried to alter their historic gene pool to make all Romulan men stronger...which was moderately successful, but led to diseases wiping out 50% of their population due to the genetic changes, as well as making Romulan women smarter, overall. These poor Romulans are just falling apart- what with them losing 1/3 of their population previously due to space COVID...I really feel for them...and still wonder in what way they were ever a threat...
Should you read it? Well...um...I dunno...maybe?