Welcome to I Find Dead People.
Right now, I'm mid-way through the second volume of Cumberland County, NJ, Letters Testamentary. I copied the entire book digitally at the NJ State Archives in May. I have been entering next of kin information for anyone whose name I recognize (which is pretty much everyone). This includes birthdates, death dates, marriage dates, children, etc...
Letters Testamentary is a piece of paper filed with someone's probate when they die. It lists their next of kin and town they're living in (now it lists their addresses). This is especially helpful when you have someone born before 1848 (when NJ birth certificates started) who has no children. Why?
Because it will list their siblings, and if you're especially lucky, cousins. The best is when those cousins are dead and it lists their next of kin. I was able to fill in 5 Bateman cousins from my 3rd cousin 6x removed David L. Snyder (1823-1886). It mentioned his aunt Margaret Bateman Dixon Hall and her children living in Ohio, among others. Sometimes people migrated West before the detailed household censuses began in 1850 and you have no way of confirming their identities because that state didn't start vital records until much later.
I also found out my 3rd great grand aunt Ruth had 2 children out of wedlock before marrying my uncle. That's two children with different men. Scandalous!