Math is easy to find (to an extent): <a href="http://tutorial.math.lamar.edu/" rel="nofollow">http://tutorial.math.lamar.edu/</a><p>Other subjects are a bit more difficult and scattered, for physics hyperphysics does well<p>But I am indeed talking about advanced topics which are available in abundance for computer science and programming on the internet but when it comes down to it that is the extent which the internet provides (except maybe graduate papers, but that is usually too high a level to matter)<p>Say for example you wanted to learn thermodynamics with only the internet and a teacher.  You can gain a large amount of theoretical knowledge from the teacher but without a book it will be hard for you to find examples on when/how to use integral and differential energy and entropy balances (Thermo might be a subject just on the boundary of finding information on the internet and having to buy a book).