These links will take you directly to the portions of databases that contain materials relevant to Mathematics. However, this is by no means an exhaustive list. Feel free to explore the General Collections to find additional resources.
The Open Textbook Library provides a growing catalog of free, peer-reviewed, and openly-licensed textbooks. The Open Textbook Library is supported by the Center for Open Education and the Open Textbook Network.
Bookboon.com contains free eBooks and textbooks that can be downloaded as a pdf, no registration needed. These books cannot be altered, but can be freely printed, shared, or embedded in non-commercial ways. They are intended only for individuals, colleges, and schools.
The Global Text Project is a joint initiative between the Terry College of Business at the University of Georgia and The Daniels College of Business of the University of Denver to publish eBooks that can be accessed by all, especially the developing world.