Maine Postal History and Postmarks by Sterling T. Dow is more comprehensive, but very dated.
Maine is one of the slower states for postal history.
Beyond a small hardcore group of collectors, there is little demand. As damning as this may seem, it represents an improvement over previous years. There is a lot of Maine material available, much of it is 19th Century, and it is one of the few states left where stampless covers are both cheap and ubiquitous. It is a very worthy state to collect. Maine collectors are still scarce.