Vijay Kumar Bagavath Singh wrote:

Just a word of caution. Many free software packages undergo rapid changes, for example Python and GTK+. If we were to print the manuals and tutorials related to these software packages, the printed book will become outdated soon. Yet another thing is that many people will not be willing to buy books for software packages that undergo rapid changes.

Documentation is seldom static and always in a state of flux. I think most buyers of books are either students or those who are fairly new to the application. Though internal implementation of an an application may change, the basic user commands remain more or less stable. The focus should be on these core commands, which give life to the application. If the books provide material which can give a firm grounding on the broad principles behind applications, with concrete examples based on a reasonably current and stable distribution of the application, the user can adapt to the changes without difficulty.