Oh, the dictionary is a great idea!
I looked these up on Amazon but in no way am I suggesting that you should purchase from them.
This next one isn’t a companion, it’s more of a literary exploration of the series. I have it, but it’s still TBR. And I can assure you, I do don’t spend nearly this much money! If you’re intrigued, go to Amazon.com and search for the Kindle version. The page will give you the “Look Inside” feature, which is a hefty sample of the book. This might help you decide whether you want to purchase (somewhere else) or not.