> For math textbooks, being “open source” doesn’t really add much value (compared to just free)
That is wrong. For one thing it allows someone to take up a project if the original author dies.
> I would urge people to not hesitate to pirate PDFs of good, non-free textbooks if you can’t afford them or find them too much of a financial burden
That is also wrong. Authors spend a significant part of their lives on a book and are entitled to do with it as they like. The best way to get free access to a for-cost text is to use your library. (BTW, I write books that are freely distributed.)
That is wrong. For one thing it allows someone to take up a project if the original author dies.
> I would urge people to not hesitate to pirate PDFs of good, non-free textbooks if you can’t afford them or find them too much of a financial burden
That is also wrong. Authors spend a significant part of their lives on a book and are entitled to do with it as they like. The best way to get free access to a for-cost text is to use your library. (BTW, I write books that are freely distributed.)