Look in the book for what Johnston gave as the source of the document. He might have copied from another book, or he might have found a document in Town records or at the State Library.
If he cites a document as source, it still existed when he copied it.
Even if Johnston copied from some other book, the document may still exist in Town records or in the State Library. You would have to look for it in person (or hire a researcher to do so for you) if the source-publication did not give a specific location of the record (much more specific than "in Connecticut State Library").