Zafar Ali

Check out

https://www.pdfdrive.com/category/48

https://www.servicenowstar.com

http://www.gutenberg.org/browse/scores/top

One boy I met has downloaded hundreds and only reads these. 

http://www.gutenberg.org/browse/scores/top

I want a site or repository which gives access to books related to different fields including social sciences, physical sciences etc..

many file sharing website may give you opportunity to download full book provided you need to google that. However, that may be copyright issue. 

Zafar Ali

Check out

https://www.pdfdrive.com/category/48

https://www.servicenowstar.com

Follow

You could try this:

" The OAPEN Library contains freely accessible academic books, mainly in the area of humanities and social sciences"

http://www.oapen.org/home