This book is perhaps slightly undersold by it's title, it's a pretty solid tome, still portable I suppose but it must be a good 2 or 3 inches thick. The second part of the title is also a little misleading, the majority of the authors are indeed atheists, but not limited to the more militant kind one might expect Hitchens to choose. There's a broad spectrum of Humanist, Secularist and Rationalist writing spanning from Lucretius and Spinoza to Ibn Warraq and Sam Harris. The book progresses through these in a roughly chronological order charting the way human thought on the divine (or lack thereof) has changed and progressed.
The readings are well chosen and Hitchens provides a little introduction and context to each section (if I had one minor complaint it would be that these intros could have been even longer, they were fascinating in their own right). He also provides an overall intro to the book as a whole.
If I was to direct someone, atheist or theist, to a single book to explain non-theistic world views to them, it would have to be this.