I'm writing lecture notes for one of my exams, both as a process to learn the subject well and to be able to share it with other students later. The official lecture notes are just slides and are really vague, would be/is of no help for anybody who didn't attend the classes (like me).
There is a huge book that describes many things I'd need to know for the exam. I'd like to know how much I could quote from the original book for my the lecture notes that I'd later make available online. I would not sell my lecture notes. I'd, of course, cite everything to the best of my knowledge.
I'd like to share the final PDF with other students after I finished the university (in a couple of months). I even thought about sharing the TeX source with the professor, so he (or his students in the future) can keep the lecture notes up to date.
The original books are +2K pages, my work would be around 30-50 pages, tops, with many other sources, if that makes a difference.