CategoryOnlineBook is used to mark WikiPages about books that are available online--meaning the entire text of the book can be read online or downloaded free of charge (but see note below). Simply being for sale on http://www.amazon.com or elsewhere doesn't count.
Click on the header to get a list of the WikiPages describing the books.
Note that the online editions of many of these books come with use restrictions--such as non-commercial use only. One even has a restriction that if you print the .pdf files, you owe the publisher US$.02 a page for each page printed or copy made. (In all cases, you're pretty much on the honor system, I suppose.)
In most cases, a print edition of the book is available, which Amazon or Powells or whoever else will be happy to sell you. Some of the books are online-only; and some are "draft" versions of books still being authored/revised (and might become unavailable online once the completed book goes to press).
Also see CategoryBook.