You can download books from various sources:

1. NPTEL & NDL (National Digital Library) – Free academic books.
2. Google Books & Open Library – Some books are available for free.
3. Z-Library & Library Genesis – Large collection, but check legality.
4. Project Gutenberg – Free public domain books.
5. Official Websites – Many publishers provide free PDFs for educational use.
6. Telegram & Reddit – Some groups share book PDFs.

Always ensure you're accessing books legally to avoid copyright issues.