The book "My Beckwiths" was published by a Beckwith who lives in Vermont (Bennington, I believe). It traces one line of descendants of Matthew of CT only. This line includes Oscar FitzAllen Beckwith (famed murder of NY). I can access the address of the author. If you need the address, please e-mail me at email@example.com so that I can send it to you personally.
Unfortunately, the author did not provide documentation.