The purpose of this License is to make a manual, textbook, - or other written document "free" in the sense of freedom: to - assure everyone the effective freedom to copy and redistribute it, - with or without modifying it, either commercially or - noncommercially. Secondarily, this License preserves for the - author and publisher a way to get credit for their work, while not - being considered responsible for modifications made by - others.
This License is a kind of "copyleft", which means that - derivative works of the document must themselves be free in the - same sense. It complements the GNU General Public License, which - is a copyleft license designed for free software.
We have designed this License in order to use it for manuals - for free software, because free software needs free documentation: - a free program should come with manuals providing the same - freedoms that the software does. But this License is not limited - to software manuals; it can be used for any textual work, - regardless of subject matter or whether it is published as a - printed book. We recommend this License principally for works - whose purpose is instruction or reference.