Avoid sites that ask for your or credit card to "verify" a free download. These are often scams. Stick to known educational portals or official app stores.
Highly customizable. It allows you to connect to various cloud services and OPDS catalogs.
A long-standing repository for educational materials. While the design is dated, it hosts many PDF and EPUB versions of textbooks.
I can then provide direct links to the specific textbook sets for that grade.
The iPad is the ideal device for textbooks. Use these apps to read your EPUB files: