I found The Dark Between the Trees very absorbing, but in the end a little disappointing. Not because of the rather open end, or because the mysteries weren’t explained, but because I felt that I was pitched a story where some kind of genuine investigation would be done — at least some semblance of a rigorous, academic approach to this strangeness.
Unfortunately, the character who seemed set up to do that turned out to be driven mostly by compulsion, and when she reached the heart of the strangeness, she… stopped. It makes it a mildly interesting study of the character of someone inexplicably driven, wrapped up in the mystery before they even came into contact with it directly, but I found it a little disappointing that her academia was nothing but a veneer.
The interweaving of the two stories, the modern team and the ragged band of Parliamentarians, is done quite well and used to good affect: each story reflects and mirrors the other.
I felt like the descriptions of the forest and the way it directed people inwards, to the heart of the forest, had much borrowed from Tolkien’s Old Forest. The concept of the two forests existing simultaneously, no, but the general mood, the evil at the heart, the rising fear and tension… and the ways the forest is creepy, the seeming movement of the trees, the root deliberately popped up to trip, the bracken and undergrowth through which you can fight only one way… I wouldn’t have been entirely shocked had the story suddenly involved Old Man Willow, somehow. That section of The Lord of the Rings is one I always found rather fascinating, even if I could do without Bombadil, so this isn’t a complaint, merely a note on the tone and similarity!
I did start to notice a number of editing issues in the latter half of the book: words missing, mostly, and in some cases (and I wasn’t entirely sure if it was deliberate or not), a line break where it didn’t feel there ought to be one. The author didn’t use these elsewhere and they didn’t feel very impactful to me, so I think they were just outright errors.