"The Book of Dow" by Robert P. Dow, 1926, has long been out of print. Most genealogical libraries have a copy, as do large public libraries like Boston Pulic Library. You might try the used book section of amazon.com or other used book dealers. If there is a specific listing I would be glad to look it up for you. Incidentally, there is no Down listed in the book but there is a Betsey Downs (or Downer) who married John Dow in NH in 1805.