Nye-ahm-bay
I don't have the book here at my desk so I can't give you the phonetics they use, but it's the correct pronunciation. Nearly every name in the book uses a very standard way of pronouncing things (and a pronunciation is provided the first time any such word is used).
The full name is Nyambe Tanda (tahn-dah), though in an earlier draft is was the more accurate Nyambe Taanda (tah-ahn-dah). They simplified it to make it easier to pronounce.