This is not a bad book, but it's not as good a book as it could and should have been. There are certainly good parts to this book, but overall it's just not as good as the host of positive reviews I've seen may lead you to believe.
The plotting was tedious at times. The goal of finding the seventh book isn't mentioned till several chapters in, then it is forgotten while the moles go and do other things. Then at the end, it almost feels like he said "Oh, yeah. The book. Better get back to that." There were several parts where the characters seemed to be just hanging about waiting for something interesting to happen. The length could have been cut by a third and the novel probably would have been better.
Duncton Wood is a first novel, and it shows.
For example, there is a lazy "world building" technique that mediocre fantasy authors sometimes employ; they take something familiar and give it a different name. A rabbit might be a long eared hopper; a mile becomes a footday; a month becomes a moon cycle. In this book, it took the form attaching the word "mole" to everything. Moledays, moleyears, molemonths, molefeet, molemiles, molethises, molethats, whatchamamoles, and whoozitmoles. It all got a bit silly. Were henchmoles and guardmoles really necessary? It got distracting to the point of annoyance. Some people, I am sure, found this quite endearing. I did not.
The writing frequently demonstrates Horwood's lack of faith in his narrative abilities. For example, he frequently switches in a future omniscient voice to emphasize the importance of some even he is about to describe or just has described. We are told, for example, the Bracken's navigation of the Ancient System is an astounding feat still remembered by moles in the future. Apparently, he believed his writing didn't convey the importance and drama sufficiently, so he had to stop and tell us "this was really important!" It's sort of like telling someone "That was a really funny joke" after you've finished telling it. The person laughed or they didn't, insisting it's funny won't change that. Similarly, either the writing is strong to convey the drama, suspense, and emotional impact or it isn't. Don't tell me an event is important, show it. Don't tell me Rebecca is wonderful, show her being wonderful. Don't tell me Boswell is wise, show him being wise. Instead, Horwood falls back on explaining how you're supposed to feel about events and how we're supposed to think about characters. This is a serious flaw.
The characters are too often mere stereotypes. Hulver is the wise older mentor. Rose is the selfless healer. Medlar is an eastern martial arts master. It's okay to have some stereotypes in a strongly plot driven book, but this one isn't strong enough to keep you from noticing that so many characters have little to no depth.
If you are fond of Watership Down and hoping for a similar experience, you won't find it here. The rabbits in WD were rabbits, not humans in rabbit guise. The story they are engaged in is one which a group of talking rabbits could plausibly undertake. The moles here are humans. Sure, they have poor eyesight and eat worms and beetles, but they are simply small, furry humans. This isn't necessarily a criticism, just an observation. (yes, I am aware that rabbits don't talk. Still, the point stands that Hazel, Fiver, Bigwig, and Dandelion felt like rabbits and Bracken, Rebecca, Mandrake, Boswell, and Mekkins feel like human beings).
Yet, in spite of my complaints, I repeat, this isn't a bad book. It's okay, if a little too slow in parts. It's just that it felt that with more confident handling and sharper editing, it could have been so much better.
Obviously, a lot of people loved this book. I'm pleased for them and for Horwood. I just feel this book is not nearly as good as the positive reviews make it out to be.