Many people curious about their roots expect to begin and end their search quickly, hoping to find their family tree already done. It doesn't often happen, but both published and unpublished family histories can be found at public libraries, in the collections of local historical and genealogical societies, and on the Internet. Try a search in the Library of Congress and Family History Library catalogs to see if a family history has been published on your ancestors. Online, Ancestry.com offers subscription-based access to digitized copies of thousands of published family histories. For free sources of thousands more online family histories, check out 10 Fabulous Sources for Family History Books Online (my favorite? Hathi Digital Trust).
Even if you're one of the lucky ones who finds their family history already researched and published, there is still more work to do! Begin by reviewing all published genealogies carefully, as most contain at least a few inaccuracies. Do the dates make sense? Are there a lot of typographical errors? Do most facts include a source citation? Were a wide variety of sources used? If the book doesn't include sources, then take its statements of fact with a grain of salt!
If the book is well-documented and you're comfortable with the author's research, there is always still more to do. No family history is comprehensive - there are always additional generations, other surnames, or additional sources to pursue that aren't covered in the book.
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