National Digital Library, https://ndl.iitkgp.ac.in/ is the most comprehensive resource to access free educational material. Be it any subject, any standard, any level or any format (book, video, audio, etc.), the NDL site provides all the answers. The project is a very prestigious project of Government of India and is supported by MHRD (Ministry of Human Resource Development) and IIT (India Institute of Technology), Kharagpur
Don't look any further and start accessing the free repository of content - text books, reference books, subject videos, lecture videos/ audios. The beauty of this project is that it goes beyond the typical academic content and offers content much broader in range, going upto the research areas.
Just go ahead and start downloading !