I'm not sure exactly but when I took an unexpected dive into understanding solitons in some work I was doing, I found this book to be a great introduction http://www.amazon.co.uk/Physics-Solitons-Thierry-Dauxois/dp/0521143608/ref=sr_1_12?s=books&ie=UTF8&qid=1424809934&sr=1-12&keywords=Solitons