This is not the novel I was expecting. From my interpretation of the blurb, I expected to read about Kirk and company galavanting across the ages in the timeship Enterprise. This is not what happens. Forgotten History is a story about time travel, and it certainly contains time travel, but it is not a time travel story. Those who have read the earlier DTI novel Watching the Clock (or possibly just seen 300 Days of Summer) should have a good idea about what I mean.
Forgotten History works very well as a Trek novel. The scenes and characterization feel spot on. The story takes place across ten years of TOS history. It involves many episodes (including several from the animated series) and references several other novels, but is definitely capable of standing on it's own. Similarly, it serves as a follow up to both Ex Machina and The Darkness Drops Again, and it builds upon them without requiring the reader to be familiar with either. Best of all in my opinion though, is the material linked less directly to previous works. There is a scene that takes place after TOS but manages to capture the same feel, it's like watching an episode from season 5. There is also some excellent material regarding Spock's personal relationships. Finally, the original characters, both antagonists and TOS era DTI personal managed to feel real and true to period while subverting my expectations.
The one major weakness of the novel in my mind is the "modern" TNG era section. This portion is essentially a sequel to Watching the Clock, and serves as a frame story for the rest of the novels events. Once again, the characterization stands out, and it compliments Watching the Clock, but unlike the rest of the novel, I don't think that it would stand well on it's own. When I caught references to Ex Machina I felt like I was getting an independent perspective, that I could experience one novel without even knowing about the other, but it felt like too much would be lost from the frame story without having experienced the characters before. I think this was a result of the frame story being merely a frame story rather than a full B plot. Watching the Clock had flashbacks interspersed through it that played well of the main story and felt more balanced than the narrative here. Without more time to focus on the DTI characters, the reader has to have prior experience for them to feel as real as the TOS characters.
That being said, if the worst thing that can be said about a book is that you may have to read another excellent book to get the full effect, then it can't be that bad. I would highly recommend both, but particularly Forgotten History, and I look forward to any Department of Temporal Investigations novels that may arrive in the future.