There's four or more different books going on here: a history of Liddesdale and the Borders, a memoir/travelogue of moving to and exploring this area, a treatise on interpretation of Ptolemy's geography, and a speculation on the possible historic basis of some Arthurian history.
The problem is that they don't come together into a coherent whole, so the book is more a series of digressions. Just when something is getting interesting, it ends for something else to come in instead.
It's a shame because the section at the heart of the book with tales of the reiver families of the border is interesting and I wanted more of it.