For maths, here's a good starting point:

https://people.math.gatech.edu/~cain/textbooks/onlinebooks.h...

For CS, check:

https://www.reddit.com/r/csbooks/

For more maths, and other topics, see:

http://www.reddit.com/r/eebooks

http://www.reddit.com/r/mathbooks

http://www.reddit.com/r/physicsbooks

http://www.reddit.com/r/econbooks

For yet more on maths, see:

https://aimath.org/textbooks/approved-textbooks/