I wrote last night about the delicate balance an author faces when trying to write the second (or later) books in a series. Each book must be able to stand alone, but--at the same time--build on what has gone before.
I approached a solution from two angles: (1) I reviewed two of the series that I enjoy (J.D. Robb's Eve Dallas and Robert B. Parker's Spenser) and (2) I emailed the first three chapters of my sequel to friends who haven't yet read the first book in the series.
Interestingly enough, the results were the same from both approaches. In looking at the two long-running series, I found brief explanatory mentions when characters were re-introduced and only passing comments to backstory. These rarely occurred in the first chapter in which the action for the new story was being set up.
My friends actually complained that they wanted more backstory. Then, when I offered it to them, they complained that I was giving too much detail. So, it appears that what is needed are very brief references that help to explain the relationship of characters to each other without a lot of detail.
Okay, back to the drawing board.