Easter Egg Guide for Fiction

Have any of you created an easter egg guide for your fiction book? Or maybe you’ve received one from an author. Perhaps it was a freebie for your email subscribers, part of a Kickstarter campaign, or something else.

My question is: what’s the format, layout, length, and so on?

I’m working on one for one of my YA books and have some general ideas of what I want to put in it. But I would really benefit from a some examples or some kind of general framework. Or maybe there are no conventions!