I'm in the final stages of the revision and I'm considering adding a glossary and/or index. While I think the index would be helpful, especially if the book is to be used as a reference item, I don't know whether a glossary will be as useful.
In this day and age, most of the questions a person would have can be found on the Internet. Since most of the people buying my book are getting electronic copies, they obviously have access to the Internet. Having a glossary may not be very beneficial, especially since attempt to clarify any new or possibly difficult concepts within the text.
So, if you think a glossary would be helpful, please let me know in the comments. Otherwise, I will work on just the index and leave it at that.