Dear Mr. David Book,
Thank you for posting your request on History Hub!
A search in the National Archives Catalog unfortunately did not render any results for a Michael or Mary Book prior to 1783.
Ancestry was searched and a Michael and Mary Book were both located in Botetourt County, which is the county that Fincastle is located in. Please see those search results here:
Please note that Ancestry is a subscription-based database, but it is available for free public use at all National Archives facilities and many public libraries.
In the case that you are new to genealogy, you may wish to view the following link on conducting Genealogy Research at the National Archives, it does a great job listing topics, resources, and tips:
We hope this information is helpful and good luck with your research!