A family Bible can be a great source of information. In earlier times, all vital information was recorded here: births, confirmations, marriages, deaths, and all other life events.
When looking voer these records, be careful to check penmanship and ink entries. Some entries may have been made by a single individual all at one time [these entries are more susceptible to error]. Also check the publication date of the Bible. If that date is after any of the entries, be highly skeptical; these entries now become secondary entries rather than primary entries. [Include the publication date of the Bible in your source information.]
Check entries made in ballpoint ink and in colored fluid ink. Ballpoint pens were introduced to fighter pilots in WWII, and they were not made available to the public, worldwide, until 1946. If an entry was made with a date prior to 1946, you can rest assured that it made after the beginning of 1946. That means it is certainly a secondary source and it is less reliable than one made at the time of the event.
Fountain pens were reintroduced in the 1960's and '70's to the public with many colorful plastic ink cartridges. The presence of fountain pen entries can be another indication than an entry might have been made later than at the time of the original event.
Personally I have found more pencil entries for earlier legitimate entries than those of ink entries. But the greater amount of my research has been that of the mountain people of the Appalachian mountains, and may not be the norm for other areas. - cbh