Because people will pay for it.
Okay, if you want hand-waving reasons: because it takes time to go to a bookstore to buy it, or because people don't want to carry a book, or because it's easier to search.
Or from the economics side, because there's less competition for e-Books, or because people want to pay more for things that are intangible to make it feel worthwhile, or because there's a different sort of marketing system and expectations for online stores than regular bookstores.
But in this case I think it's because you're looking at Amazon.
At the publisher's web site, http://www.informit.com/store/framework-design-guidelines-co... , " Book / Your Price: $51.99 / List Price: $64.99" and "eBook (Watermarked) / Your Price: $41.59 / List Price: $51.99".
At Amazon, "eTextBook $51.34" and "Hardcover from $4.08 / 8 New from $36.52". The 8 new are all from third-party sellers. The highest 'new' price is $87.46 + $3.99 shipping.
So most likely a bookseller is trying to get rid of unsold stock, while there is no unsold stock for e-books.
