Well this could be an answer that gets me into trouble...both with publishers and other authors...but hey you asked
My book is part of a 'Practical Guide' Series that are a set of books very focused on the topic. Mine in particular, is focused on real world developers who have a project and/or product to get out the door, and need to come up to speed on Struts very quickly. There is also an intentional limit on the page count for the series. For those that haven't authored, trust me, it is MUCH harder to write short, concise, and to the point than it is to drone on about a topic. I had to make some very real and hard choices about the topics. In my preface, I clearly state that I'm following the 80/20 rule. 80% of what you'll need to get your application built is contained between the covers while the other 20% I give advanced feature pointers and solutions to some of them. Building a sample application that follows the book gives the reader a chance to actually work with the code while they read about it to learn.
From what I can gather from some of the book comments, people seem to think thin=beginner, so they get in over their head because they don't have the proper background. From one of the reader comments on Amazon.com:
'With a sub-price tag, it's hard to justify spending 2 and 3 times the amount on another book. If you need a true practical guide, this is it'
To answer the ‘why aren’t there more thin books’ question…the answer is simple. Publishers can’t charge $39-$79 dollars a book if a book is only 150 pages. Notice the $19.95 price tag on mine. Lower price tag also means that the author doesn’t make as much. So if it takes the same amount of time to write a 150 book as it does to write a 550 page book (and it does I can tell you from first hand experience) why would an author do so? Personal preference I suppose. First and foremost, I’m an engineer. I’ve spent a good amount of time in this industry and I know when I need to learn a new technology I don’t need (or want) a 100 page tutorial on why a JVM is platform independent. I write my books for actual engineers who have work to do. Not for the sake of taking down a lot of trees unnecessarily. I’d rather have a solid group of readers that get a large benefit from my books then a huge amount of readers who shove the book on a shelf to gather dust.
I definitely put my book head to head (and dollar for dollar) with any of the other Struts books out there and leave it up to the readers to determine if the extra 400 pages was worth their time and money