Wetlands are ecosystems that help the environment process toxins, thereby maintaining its relative health. Mitsch and Gosselink's Wetlands is the professional reference, a longstanding bestseller that continues to set the standard for books on any type of ecosystem.
The difference from Wetland Ecosystems is that this is more about the properties of wetlands as a whole, like where they are, what their chemistry is like, what people get from them and how people modify them, and so on.
I read this for my wetlands week in reading class. I did not enjoy it very much and found it extremely boring. Although it does bring up some important facts, I was not impressed with this book.