1. For math textbooks, being “open source” doesn’t really add much value (compared to just free).
2. Don’t limit yourself to free textbooks. 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 (textbooks are usually outrageously priced in the U.S.) A good textbook vs a mediocre one probably doesn’t matter for entry-level topics (think freshman, sophomore, or even junior level topics for math majors), but for advanced topics, good textbooks can be markedly more insightful than a mediocre one, or sometimes you need to approach the same topic with different mindsets by learning from multiple texts.
2. Don’t limit yourself to free textbooks. 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 (textbooks are usually outrageously priced in the U.S.) A good textbook vs a mediocre one probably doesn’t matter for entry-level topics (think freshman, sophomore, or even junior level topics for math majors), but for advanced topics, good textbooks can be markedly more insightful than a mediocre one, or sometimes you need to approach the same topic with different mindsets by learning from multiple texts.