> dual licensing under GPLv3 + commercial
GNU GPL + proprietary, presumably, you mean. But in General this practice is quite clear title — selling exceptions to copyleft
. "Dual licensing" to call it imprudent though, because different licenses may be (and likely is) much more two somebody, you may sell urgent non-free license, someone indefinite, someone with a limit on the number of cars, some without, and so on.
Also, this practice is not limited to actual third version of the GNU GPL. And surprisingly, not limited even by the strong copyleft of the GNU GPL or GNU Affero GPL; some comrades manage to sell exceptions to weak copyleft GNU *Lesser* GPL, examples: Qt, CppCMS.
What is there to read, I don't even know — practice guileless as a stump. The only thing we must attend to is agreements with future free co-authors (contributor agreements) if they want to appear. Because you to sell exclusion for the benefit of his own pocket, obviously, should have the right to do that, ideally (for you) means to be the sole owner of the program, then sponsors will need to request the assignment of author's property rights (copyright assignment). A good example of such agreements is traditionally considered ownCloud'ovsky
In addition to the three mentioned examples from different areas: MySQL (database), Ghostscript (library to support PS and PDF), ChibiOS (RTOs).
In short, if you have specific questions, it would be better to ask them.