Free Downloadable Physics Textbooks

Light and Matter
The Light and Matter series of introductory physics textbooks is designed for an algebra-based one-year survey course. The textbooks available for download are Newtonian Physics, Conservation Laws, Vibrations and Waves, Electricity and Magnetism, Optics, and The Modern Revolution in Physics.

Motion Mountain
This free textbook, with little mathematics, explores mechanics, thermodynamics, special and general relativity, electrodynamics, quantum theory and modern attempts at unification.

Calculus-Based Physics
Calculus-Based Physics is an introductory physics textbook designed for use in the two-semester introductory calculus-based physics course.

Simple Nature
Simple Nature which is another calculus-based engineering physics textbook.

Conceptual Physics
A conceptually-based physics textbook by the author of Light and Matter.

A Textbook for High School Students Studying Physics
FHSST (Free High School Science Textbooks) Physics is a free introductory physics textbook. It is a featured book on Wikibooks because it contains substantial content and is well-formatted. You can download a pdf version of the entire textbook.

Calculus
This short introductory Calculus text focuses mainly on integration and differentiation of functions of a single variable, although iterated integrals are discussed.