Before you buy a genealogy book, check here

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4. Many of the old researchers went to the locations and actually looked at old records

Which is invaluable because a lot of those records have degraded in the hundred years or so since those books were written.

What I love is that books my mother never had access to or had to wait ages to get through interlibrary loans or had to go to Washington to the DAR library are now available free on the internet. Some of the books she used for reference and only got to see once, I've found additional information that she missed when she used them. Others she never got to check and I've found more info in them.