What is the difference between the freely available chapters and the for sale printed book?
The book presents only the chapters as needed for a particular university.
The book has problem sets.
This is a way to support the authors so that they can continue to bring new topics to the readers.
The author is making a nationwide effort to show that quality textbooks can be kept free for in-need students and the general public, while still generate a revenue stream. Think Radiohead.