Rather than a single resource, I would suggest reading the Wikipedia entries of the following: I(D)MS, Ingres, PickOS, Mumps, GT.M, dbm, Datacom and ADABAS. All

Rather than a single resource, I would suggest reading the Wikipedia entries of the following: I(D)MS, Ingres, PickOS, Mumps, GT.M, dbm, Datacom and ADABAS. All of them have great historical value.