This isn't a math question, but I am interested in the following. So, I am self-learning concepts of linear algebra and I heard that Gilbert Strang's book Introduction to Linear algebra is excellent for those who want deeper understanding of the subject, but I can't find it online. Is there another safe place to search or buying is the only way? Thanks
Edit: I don't want troubles with this wonderful site so please give answers that are under rules of MSE
I don't know if it can be found for free online according to the author but you can look at this: https://ocw.mit.edu/courses/mathematics/18-06-linear-algebra-spring-2010/video-lectures/
This is the course of MIT about Linear Algebra held by Gilbert Strang in person.