Simply put, this book is brilliant. The story is well done and clearly the author did considerable research for the Second World War setting. Furthermore, it is a great read, unlike many post-HPL-circle stories. It is a true credit to the Delta Green concept. More, Detwiller's story is so well done it very well could be one of the best post-HPL circle mythos stories around and should be read by not just those interested in the Delta Green concept but anyone interested in Cthulhu Mythos stories.
Suggestion: Read HPL's "Arthur Jermyn" and "The Shadow Out of Time" BEFORE reading this novel. Detwiller mentions his love of and use as building blocks of these stories in the "Author's Note" at the novel's end, which is a shame; his modest "additional layer" to the stories shows all the better if one has read the two HPL tales before reading his novel. (Indeed, it would be nice if all post-HPL-circle stories mentioned in the beginning which stories they built upon.) Also, if you can read Detwiller's short story in the Delta Green: Dark Theatres anthology before, that is all the better. (Not necessary, but beneficial to enjoyment and appreciation of this novel.)
Hopefully, as they are great fun, Armitage House will produce many more Delta Green novels and anthologies, as well as reprint their previous releases (such as the impossible to purchase DG: Alien Intelligence). Again, these books are some of the best post-HPL-circle stories available today and are highly recommended to anyone who enjoys Cthulhu Mythos stories.