In a world where software vendors and computer book authors released the copyright to their books a certain number of years or versions after the version that book is about would be a really great world, and the originally linked web site might have a really good place in that world -- one that wouldn't require them to run more ads than a gadget blog.

Unfortunately, in the real world, no books about computers have fallen into the public domain, and if you want to get free books, a lot of the time tech publishers have a few books free and in the clear, usually acquired from the publisher's web site, such as on the Microsoft Press blog.