The Happy Atheist is Myers' first book. Readers familiar with his blog Pharyngula will know what to expect. The book deals with religion and atheism from a lot of different angles, and in essence it's a popular book written for a popular audience, using popular arguments. This is not to say that the book is simplistic, just that it isn't scholarly. If you want a preview of the book, read his essay "Planet of the Hats", which in the book is the chapter called "About the Author".
I half-expected the book (having read Myers' blog) to be much more angry than it is. In fact it's a very funny, lively, and readable book, and at no point was I bored. (In fact, I had trouble putting it down. I didn't read it in one sitting, but two.)