A Canadian OER initiative provides hundreds of free e-textbooks in various subjects: Economics, Business and Finance, Engineering, Biological and Physical Sciences, Academic Writing, Language Learning, etc.
Typical e-textbooks from BC Campus:
Developed by University of Minnesota, provides good textbooks on various disciplines including Business, Finance, Computer Science, Engineering, and Social Sciences. A good platform to find textbooks such as "financial mathematics", "organic chemistry lab techniques", etc.
Typical e-textbooks from Open Textbook Library:
High-quality, peer-reviewed, openly licensed college textbooks that are free online including: Calculus, Statistics, Biology and Microbiology, Chemistry, Anatomy, Physics, Macro-Economics, Micro-Economics, Sociology, Psychology, Accounting, Business Management, etc.
Typical e-textbooks from OpenStax: