Where can I find a good exposition on double and in general multiple sigma notations?

I am using Concrete Mathematics by Graham, Knuth and Patashnik and I understand their discussion on multiple sums (sigma notations), however, I need something a bit more concrete than their "Iversonian Identities". What is a good book where I can get a really good explanation on multiple sigma notations? No funky stuff. Just straightforward algebra. Thank you for your time and help in advance.