> I don't know how mathematics textbooks are written and how much effort goes into them
They are written in many ways by many different people.
But some of us have started to write books that are Free, in the sense that software is Free. I have a couple and in addition to making the text and the source available I also sell one of them on Amazon and it does OK (see http://joshua.smcvt.edu/linearalgebra), because lots of people prefer a paper version when they really get down to studying.
They are written in many ways by many different people.
But some of us have started to write books that are Free, in the sense that software is Free. I have a couple and in addition to making the text and the source available I also sell one of them on Amazon and it does OK (see http://joshua.smcvt.edu/linearalgebra), because lots of people prefer a paper version when they really get down to studying.