Well, that depends. Mostly I would say yes, it is. To me it was enough to own a book that said "Dothraki - A Conversational Language Course"; everything beyond that was bonus. Of course I also felt it good to have that one more source for double checking and consulting, since I'm perhaps the most active maintainer of the Wiki and go to guy for all kinds of technical questions.
Mostly I think the book is good at being a well constructed and definite source. The wiki is a mess, since amateurs like me (not even native English speaker) have built it kinda haphazardly over time. There are bound to be errors - like old stuff that never got updated. And there is a lot of stuff that's plain badly explained. It has much more information than anywhere else, but that info isn't completely dependable and takes some digging to use. David's blog is a definite and well written source like the book, but it's just a collection of posts on some semirandom topics; there is no comprehensive starter package. For the topics it covers it's the best source, for everything else it's useless.
So I guess if you want to learn with minimum frustration and start with a solid footing, get the book. But don't expect that alone make you the master of the language.
