Hello,
the GPL is not only for software. It is just that it was specifically written for software and therefore contains many ideas that are specific to software (the idea of source code, for example). There are other licenses that do a better job at protecting the author's interests for books because it uses another choice of words.
If you want to publish a free (aka freedom to read&share) book, you might want to use the GNU's Free Documentation License.
Licenses - GNU GPL, GNU LGPL, GNU FDL, General Public License, Lesser General Public License, Free Documentation License, List of Free Software Licenses
This license allows you to have finer control over which sections can be modified how.
One the one hand, sections with technical content, which may be subject to change in the future and which you usually want to be modifiable. For example, if you describe a menu item of a software and this menu item's name is changed, the developer can update the documentation accordingly.
One the other hand, you might want to have chapters in which you describe your view of the world or your favorite food. Or you may want to have your name printed on the front+back cover and advertise your website. You certainly don't want to have anybody tamper with these sections. Therefore, this license has special provisions to declare these parts 'unmodifiable'.
PS: The GPL does not a very good job at
patenting because the whole idea of software patents is at stark contrast to the attitude of the Free Software folks.
Fighting Software Patents - Singly and Together - GNU Project - Free Software Foundation (FSF) Software Patents and Literary Patents - GNU Project - Free Software Foundation (FSF)