I am a PhD Student doing outreach with High school Juniors and Seniors in STEM. There is a book on mathematical proofs, "How To Prove It" by Daniel Velleman, that I think most of the mathematically inclined students I am working with would love reading and doing problems from. These students though do not have the funds to purchase this book because they are from economically disadvantaged backgrounds. What is the most ethical way for me to obtain large numbers of copies of this book, or textbooks in general, for these students?
Contact your local library and ask them to help, they may be able to access copies through inter-library loans. If you're located in the US or Turkey, it appears that this book is available via OverDrive (https://www.overdrive.com/media/96681/how-to-prove-it) , which is a commonly used eBook lending service. It's quite likely that you have a library nearby which offers OverDrive to their clients, in which case, you'd get free access with membership and so would your students. Good luck and keep up the good work :-) ETA: I'm not affiliated with OverDrive in any way. Of course, there could be other eBook providers which have this book, just check with your library.