Zero Angel
Auror
I would include a two to three page summary of the entire book. I can summarize a chapter in a sentence if I need to. Really, you're stripping out all the characterization and emotion and relating just the facts. That doesn't take a lot of space.
How about once you get to Book 3 or Book 15? This is one of the nice things about having a glossary at the end though. You don't have to include stuff and if people don't remember they can just look in the back. In fact, I think I can sell myself on the idea of having a "road so far" synopsis as an appendix to future books.
I'm steadily moving more towards the include-everything-and-the-kitchen-sink mentality when it comes to book extras. Timelines, maps, synopses, character profiles, drawings. Unfortunately that doesn't translate well to print unless you have an ultra-super-special edition version.
Going back to the OP, what do yins think about having new words as hyperlinks to the glossary during their first appearance? Or should this just be something you do on shelfari and hope people highlight enough to enable Xray on your book?