The Blackmar-Diemer Gambit (2011) by IM Christoph Scheerer is a comprehensive, 336 page work published by Everyman Chess. In the preface, Scheerer admits that he has never played the BDG, yet sees this as an advantage in that it leads to greater objectivity. After reading much of this book, I agree. It does appear that IM Scheerer evaluates all major lines of the BDG (and many minor ones) in a fair and objective manner.

The author cites an extensive bibliography, although he omitted one fairly significant source, NM Charles Diebert's The Blackmar-Diebert Gambit (1991). One surprise, at least to this reviewer, is that Scheerer views the most critical line against the BDG as the Ziegler Defense (that is, 1 d4 d5  2 e4 dxe  3 Nc3 Nf6  4 f3 exf  5 Nxf3 c6). I assumed it is the Lemberger Counter-Gambit (i.e., 1 d4 d5  2 e4 dxe  3 Nc3 e5). However, I defer to the author's judgement on this point.

One minor, but annoying, error is a typo in the very first paragraph of the preface. Fortunately, this seems to be an anomaly. 

In sum, Scheerer's BDG book is worthy of a place alongside that of Rev. Tim Sawyer's Keybook II in any BDG player's armamentarium.

Just got the book, it looks very good... now I need to find the time to actually read it!