When our authors ask how long their book should be I always say: Long enough to cover the subject, short enough to keep it interesting. My company is called No Starch Press for a reason. Think of the word starch as a nicer way to say "BS", as in No BS Press.
There's a lot of work behind these pages as with all of our books. Unlike any publisher in this field we have several people who read and craft every line of every book as necessary, together with each author, before a book goes off to a copy editor. That's where most books start but not hours.
The real cost in creating a good book is not in the paper. It's in the time it takes to actually craft the words.