Bygdebok - literally village book - is the Norwegian genre of detailed local history that almost every kommune (municipality) in the country has commissioned at some point in the last 130 years. The genre emerged in the late 1800s as part of the national-romantic project to document rural Norway before the parallel forces of industrialization and emigration erased it, and the writing of new bygdebok volumes continues today. Most kommuner are now on their second or third edition, with separate volumes for the gårdshistorie (farm history) and the kulturhistorie (cultural history).
The structural feature that makes a bygdebok extraordinary for genealogy is that the farm-history volumes are organized by gård (farm), not by surname. Every farm in the kommune gets its own chapter, identified by gnr (gårdsnummer, farm number) and bnr (bruksnummer, sub-holding number). The chapter lists every household that lived on the farm in succession - birth and death dates of each member, marriages, occupations, and notable events including emigrasjon to America with the ship name and destination state where known. A Norwegian-American researcher who can find the gnr/bnr in an emigration record can read the family's full pre-emigration history on a single page.
Surnames in pre-1900 rural Norway were farm-derived and changed when people moved between farms, which is why bygdebok-style farm-anchored organization is more reliable than surname-anchored organization. A man born Knut Olsen Bjørge on the Bjørge farm and married onto the Berge farm would appear as Knut Olsen Berge in later records; both entries are in the bygdebok, indexed by farm.
Obtaining a bygdebok from the United States: most are out of print but available used through antikvariat.net at $40 to $120 per volume. The Vesterheim Genealogy Center library in Decorah, Iowa, holds an extensive collection on loan for members. Many Norwegian kommune libraries will scan and email a specific farm chapter on request if you write ahead in English. The bygdebok is dense, in Norwegian, and uses dialect words and abbreviations - but the format is repetitive and a one-hour orientation from a Vesterheim genealogist makes it workable for non-Norwegian readers.