Family bibles can contain much useful information although they can sometimes contain errors or even conflicting data. I have scanned my aunt's copy of a BUTT family bible that has been passed down through several generations. Frances Charlotte BUTT was my great-grandmother and she married James Edward Henry SLADE in 1899.
It is clear that various people added new details from time to time, and sometimes someone would compile a complete set of family information. This wouldn't necessarily be consistent with the previous details! But there are details that you wouldn't get from anywhere else, such as the times of the births so that we know which twin was oldest.
I eventually worked out the details as detailed in this Word document: FamilyBible3.doc
There is information on 3 generations of the BUTT family including Frances' famous cousin, the singer Dame Clara Butt.