Bygdaboks are a source unique to Norway but similar to one found in the United States, as well. Part of their contents give historical information concentrated on a parish, similar to that found in town and county histories in the United States. They are unique, however, in the fact that they put forth detailed histories of each farm within the parish, covering topics such as geography and geological features, economy, society, and politics. They also often tell which members of the community emigrated to the United States. The compilation totals about one thousand volumes and gives details for over 550 Norwegian communities, arranged alphabetically by either county (fylke), community (kommune), parish (sogn), or locality (sted), depending on the volume.
Genealogists will likely find the most attractive parts of the bygdabok are the genealogies of families who resided there, both landowners and renters. These genealogies are a very important tool that can be used in Norwegian research, as they are great place to find names, dates, and places that may have been elusive. Occasionally, the genealogical information they contain can help you trace your ancestors to the early 1600s.
Another reason they can be helpful to genealogists, too, is that these books put forth information on your family's neighbors, as well, which may prove very helpful when you hit a brick wall and have no alternative but to research the neighbors who interacted with your family.
The bygdabok is really a gold mine of information. While you may be excited to find the data you need within its pages, you need to remember that it is a secondary source. The information recorded there was collected from various persons who may or may not have had firsthand knowledge of every family on the particular farm and their genealogical history. They may have received their information from faulty sources; so, although it is a great place to supplement Norwegian research if one is available for your farm, it should not be the foundation and sole source of your research. You will definitely want to verify the information you get from a bygdabok with more primary records.
Because bygdaboks are so helpful in Norwegian genealogical research, many libraries have worked on collecting as many of them as they can. The Family History Library in Salt Lake City has managed to gather a very large number of them, and you can also find substantial collections in university libraries located in states like North Dakota where Norwegian immigrants settled in large numbers. It is easy to find these libraries using a search engine on the Internet.