It is a real, published, book and can be cited as such. I'm not sure if I should.
The book is a fictionalized story, claiming to be based upon family tradition and supported by systematic research in the church records. The main point, however, has been carefully constructed as undocumented because of cover-up.
Research in church records and other documentation cannot have been all that thorough. There are blatant errors about easily available facts, even in the immediate family of the protagonist: things like a sister being present in a supporting role five years after this sister died, while the other two, living, sisters do not get mentioned.
The "sidekick", playing an important role in the story, is a completely invented person, with a fairly ordinary name that happens not to have been used at all in those parts at the time.
I'm not going to refute the book point by point. Should I mention it at all? What do you think?