I'm a bit concerned about the amount of content covered in the Hal Leonard book. It only has 144 pages of material vs 360 pages covered by the "For Dummies". In addition, I've seen sample pages of the book through Amazon Online Reader and it seemed very brief in it's wording. Yet, many are still recommending it over "Bass Guitar for Dummies".
Is there content the "Dummies" book is covering that the Hal Leonard has nothing of? Am I missing something?
Yes, you're missing the fact that two books have different purposes. The Hal Leonard book is intended solely as a tutor book, and it is certainly one of the best out there, whereas the Dummies book tries to cover more ground in terms of reference material.