George Cain from the Georgia Institute of Technology maintains a website called Online Mathematics Textbooks, which lists dozens of popular math books. Specifically, the site contains a list of close to 80 links to math textbooks that people can access online.
While many print math books traditionally tend to be both heavy and costly, the math texts referenced on Online Mathematics Textbooks are neither, which increases accessibility.
Examples of books available, as of 2015, include the following: "Calculus" by Gilbert Strang, which can still be bought in print format, and can also be picked up electronically courtesy of MIT's OpenCourseWare. "Linear Methods of Applied Mathematics," which comes courtesy of Evans Harrell and James Herod, and "Algebraic Topology I," written by Prof. Allen Hatcher from Cornell University, are two other online mathematics texts referenced on the website.
Another resource for popular online math textbooks is the Open Textbook Library website. Examples of books listed on the site include the following: "A Computational Introduction to Number Theory and Algebra" by Victor Shoup from New York University, "A First Course in Linear Algebra" by Robert A. Beezer from the University of Puget Sound, and "A First Course in Linear Algebra" by Ken Kuttler. There are plenty of options on the site, and users can also use the reviews to ascertain which texts they might want to use.
The aforementioned are just two examples of websites that people can use to find popular online mathematics books.