In part, this book suffers from bad marketing. The blurb from David Drake identifies it as "vivid Lovecraftian horror" but the novel itself is something else entirely: an action/adventure story with some Sword & Sorcery elements, more akin to the works of Robert E Howard than of Lovecraft. There's nothing wrong with that; it's just not what I had sought.
In broad strokes, the story is engaging enough. The Ninth Legion at the fringes of Britannia face both human foes and hordes of monsters known as the Greymen. The Greymen are fairly conventional monsters, akin to Bigfoot and Yeti (and thus familiar territory for co-author Eric Brown), but they in turn are controlled by a Pictish sorcerer who serves as the primary nemesis.
The writing, however, distracted me more and more as the story progressed. A certain degree of unevenness might be expected in a book written by two co-authors. I found that this book sorely needed another round of editing. The text was rife with the kinds of errors that spellcheckers will never discover: substituting scrounged for scourged, arched for ached, close for clothes, and inane for insane. In other cases, this reader was unable to decipher what they really intended. On page 321, they wrote, "with true purpose in his steps, trite and fast..." Clearly, they meant something other than "trite" in this context.
Beyond spelling, the book suffers at times from passages with troubled syntax, and also with uneven dialogue. At times, the dialogue becomes too modern, as in this sample from page 163: "Well, ya got me there, sister, or Queen, if that's what ya really are now that yer here." The speaker of that line is said to be a descendent of the Spartans, serving in the Legion. Other anachronisms jumped out at me, but many of them are unlikely to be noticed without fairly specialized historical knowledge.
Dedicated action/adventure readers may well overlook such flaws. Indeed, fast, informal prose and irreverent comebacks are probably selling points for many.