The UC Berkeley has made arrangements for electronic copies of certain textbooks—including the one for this course—to be accessible online through the HathiTrust. Here is some information about the program. Here is a link to Rudin's Principles of Mathematical Analysis, Third Edition.
The Natural Number Game - an introduction to Lean, an open source theorem prover. This is a "game" which involves using Lean to construct the basic theory of the natural numbers from the Peano axioms.