"There seems to be a lot of information about reincarnation, but not so much on the soul in-between incarnations (so to speak). This is what I'm really curious about"
Although I have not read them yet I am aware of two books written by a master hypnotherapist, Michael Newton. They are the "Progression of Souls" and "The Destiny of Souls" He found in his regression with patients that they were able to tell him what they were doing in between lives.
Apparently we have soul groups or clusters of intimates that we arer associated with in and out of incarnations. Might be interesting to check out.