The few times I've e-mailed mine, I've generally just said, "You don't need to write a detailed response to this, but I'd appreciate you just letting me know that you've read it." Once she called in response, the others she wrote something brief. The same with phone calls/voice mails--sometimes I'll say I'm just leaving her an update as an FYI, other times I may ask her to call me back if she has time.

I wouldn't send a long e-mail as the first one, though, unless you've discussed it with her. I'd also suggest writing things down in between sessions that you think of and want to talk about in between sessions. I find that helps me remember stuff that I want to address and stay more focused.