Its absolutely true. The exact same effect occurs in restaurants and floral arrangements.

There is a fixed cost, we'll call it X, to get the {book/meal/arrangement) made, the marginal cost of {paper/food/flowers) is small relative to the fixed cost and yet is the dominant factor in perceived value.

Understanding this you can use it to your advantage. You can boost the percieved value of a book or meal or floral arrangement by boosting its pages, vegetables, or flowers. The recipient will feel better about getting and the buyer will feel better about paying for something with a higher perceived value.

Wow, great answer. I would love to read more about the psychology of this. Does it differ across cultures?

I suspect it's a perception issue... the price difference between a slim programming book and a fat one is minimal. Dropping $50-$60 on a tome feels like a better value if you have to wheel it out with a dolly, instead of slipping it in your pocket.

I have to concur, unfortunately. If a tutorial book adds a huge reference section, I'd feel that it's a better deal for the same price- I may never look at the appendices, but I'll feel that I possibly could, just in case. Publishers ought to fix the pricing in general.

regardless of whether it's true, greenspun reported that it's what the publishers believe:

"It is an article of faith in the computer publishing that bigger books sell better. They take up more space on the shelf and readers with a tech problem find a bigger book comforting. Readers aren't really sure what is wrong with their computer and they only have a few minutes in the bookstore so they figure the thickest book is the most likely to contain the solution."

http://philip.greenspun.com/wtr/dead-trees/story

Also consider that on a bookstore shelf, the bigger the book is, the more space it has to advertise itself. It is its own advertisement.