It's something I never actually thought about before now, how to decide what happens in the empty space between the last page of one book and the first page of the next. I'm in that position now, while I give myself some clear head-space after finishing the first draft of book one and wait for some feedback from people who've offered to proof-read for me.
On the one hand, it's daunting because I need to consider how events will have shaped my characters and how they'll be acting and living their lives over a longer period of time, without the benefit of being "on the ground" as it were, in their heads. On the other, it's useful to be able to advance events to the point where I need them to be for the sake of the story.
I'm currently compiling a list for each character detailing what's been happening to them. I'm planning to maintain this even for characters who might not appear, so that when they do return, I have a reference point to re-introduce them.