Hexanaut.io appInstall Hexanaut.io and play fullscreen
Tap Share 
and Add to Home Screen 
and Add to Home Screen Which are you following (Russia, Kazakhstan, Ukraine, etc.)?
Often provides links to federal lists of approved textbooks.
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.
The native app. It handles EPUB perfectly and syncs across your iPhone and Mac.
