Last night I was listening to a podcast by Joel Spolsky and he had a nice little story about how computer books [Excel Made Easy, etc.] came to be so fat.
Apparently, publishers found out that readers were buying such books mainly on their thickness. If somebody wanted to learn how to use Excel they had no way of knowing which book might be best, but were inclined to buy the thickest book in case it contained more useful stuff.
Publishers started to press authors to add extra chapters and lots of screen shots to books to fill them out. The result was an obesity problem amongst computer manuals. Instead of people being offered the slim, concise and cheap books they really needed, they finished up only being able to buy fat and expensive books that were full of padding.
Spolsky also said the computer book industry grew out of piracy. People illegally copied programs, and then needed manuals to know how to use them. It used to be that Microsoft and other software firms provided excellent guides to their programs. Then it all went online to reduce costs/increase profits [delete the one you think does not apply]; but online help is not as good as having a well written manual.