If you have prisoners in your family records, their lives may be a very rich source of genealogy for you, depending on where they lived.
The following is information that I have been able to obtain from these records, I hope that you are able to do the same.
Names and addresses of friends and family members (and possibly the relationship to the prisoner in court papers and newspapers!) On a relative that was charged with murder, I was able to confirm the names of his parents as well as a branch of the service that he was in before the incident.
Type of repeat offender. Court papers referred to this person as a repeat offender. I did not know that the crime he was incarcerated for was not a "One time thing". That information led me to follow the records and addresses of that particular branch of my family. Without the prior arrest, I merely would have had a gap in residences with no reason why.
Warrants of arrest may also point to different places that the person was living at or committed crimes in. My relative had a warrant in a larger town miles away for something that was completely unrelated. His last known address (as was listed on the warrant) was in the city I was born in. This warrant was mentioned in the court documents, and I was able to look for any activities there in the city. I also had the address to look at, which was the part of the city that my family lived in.
Money and letters. Letters from this man beseech my family to send money for his "canteen" account. He had just gotten money in one of the letters, but the prison sent the money order back to the family because it was over the maximum amount that was allowed in one transaction. In this letter, my relative offers suggestions of relatives to go to that have an understanding of how to get a money order that follows the rules set by the prison. He then admonished the sender to not send such large funds as they will draw unwanted attention to him.
In another letter, he made a list of who he wanted to send Christmas cards to. That letter was a wealth of information, because some of those people, I had never heard of before.
Mention of addresses and family affairs such as children, property, marriages and divorces. My relative was incarcerated before he was 35. He sent numerous visitor request forms, because the prison that he was in was very picky to make sure that everything was filled in correctly. During this time, he had to send a letter to this person underlining the correct family relationships. That was family tree gold to me.
Special "leaves" from jails and prisons for funerals of close family relatives. During his time in prison, his grandmother died. He was taken to the funeral, where he was heavily police escorted the whole time, driven there and back. I remember this event as being pretty sad and traumatic for me. I did not expect to see him at the funeral, let alone in leg irons.
Prisoners buried at Prison Cemeteries. For some prisoners, their families have either moved on without them in their lives or they are unable to afford the cost of transporting the family and paying for a funeral. If you are able to get a copy of their death records, they may list the next of kin and other information. My family member was not buried at the prison. Instead, he was cremated and his cremains were sent to his family. I am in the process of writing to see what records will be released to me, if any.
Online Prison Records. These sites may include inmate photos of their booking photo.
Extremely violent or shocking crimes may be printed in more than one newspaper and may not be confined to the local area.
Outcome of trials and hearings. Sometimes, the outcome of the trial can lead to further information. In the information that I had about my family member, it was mentioned who was in the courtroom when the verdict was read. This was very useful information. I learned of his sentence and where he would be incarcerated at. (Even though the events I was reading about were decades old, it was very helpful to me to be able to follow his path in case there were other places of incarceration to look into.)
Hospitalizations and illness. Medical records may have vital medical records information. My relative was in and out of the hospital all of the time for a lot of similar issues. It was helpful to me when I was writing about my medical information to be able to include the knowledge that I was able to glean from medical records.
Good Luck in your search and also know that certain sites online have a lot of good information and records of incarcerated people. I will be including those links in an article this winter.
Have a great search and let me know how you found this information to be helpful to you.