Why would you spend $10 to learn to use Basecamp?

This is sounding very strange to me indeed! 37Signals is very much known for creating the simplest of interfaces and I really found people finding it very intuitive. But this is strange... now they are asking people to buy a book for $10 to learn how to use Basecamp! Can you believe it? See the sales post (!) here at "Sams Teach Yourself Basecamp in 10 Minutes" is a comprehensive guide to Basecamp