An index would be really useful, but given it's meant to be carried around, why not put a page number next to the words listed in the mini-dictionary at the back?

Sometimes what you want to look for does not fall under a distinct category and it's hard or even impossible to find what you're looking for especially if you need to find it quickly. So if you could look up the word at the back and then be directed to the page number/s of the section/s it appears in, then you'll be able to find related words/phrases.

HOpe that made sense.