Many regional governments have "Electronic School" portals (like MES in Moscow) where students can access books for free via their school login.
A long-standing repository for educational materials. While the design is dated, it hosts many PDF and EPUB versions of textbooks. epub shkolnye uchebniki dlia ipad skachat besplatno
Great for classic literature required in school (English and some Russian titles). epub shkolnye uchebniki dlia ipad skachat besplatno
I can then provide direct links to the specific textbook sets for that grade. epub shkolnye uchebniki dlia ipad skachat besplatno
The native app. It handles EPUB perfectly and syncs across your iPhone and Mac.
A prompt will appear. Select "Open in Books" or "Save to Files."