Start by using the RootSearch tool on her profile. I just took a quick look and her birth and christening record are on FamilySearch. Surprising for a record that old. The tool will also give you other options of places to search.
As for the book, you do have it cited properly. Does it contain information that directly refers to Sarah? You only use it for her if there is information about her in it and there is no primary source, or as a second source in addition to a primary source. The link needs to be created from the page you are citing. That way when people click the link they can see exactly what information you are referring to, and not have to search the book.
Online genealogy trees are not acceptable pre-1700. You can use them by looking at their sources and trying to locate where the information originally came from. Then cite the original, not the genealogical tree.
BTW, I just noticed her birth, marriage and death records are all on FamilySearch.
This time period in the US can be hard to locate primary records. Record keeping was not all that great and then the wars and such, many were destroyed. The location she was in should not have had as many problems with that as some states.