While writing for my thesis on "VoIP for SME's", I read a couple of VoIP books. The book from Mr. Harte is very brief (and the layout/cover is not very attractive, it seems very outdated). Considering the price of the book, it has a very bad price/info ratio. If you want to read a primer, I would recommend VoIP for dummies. For more advanced reading, try Voice over packet networks from Wright (you need to know the fundamentals, before you read this one!).