While a few others have used the term verbal self-defense (or defence), to my knowledge the originator is Suzette Haden-Elgin. She's a Psycholinguist and author who has written extensively on the subject, and is in my view, highly worth reading.
Some of her books overlap, and you probably won't want to read all of them, due to duplication of some concepts, but then again, there's different content in each of the books.
Some of the techniques I teach for defusing angry customers come directly from Elgin's work, but there's actually a lot more of her work that I haven't yet "translated" to the customer service world. So, in other words, if you are sharp, you'll probably find things YOU can adapt.
Of course the books are valuable for things other than customer service. After all, they weren't intended for customer service situations, yet many many of the techniques fit well.
Below is a display of books from here, and available on Amazon.