In addition to the Free Math Ebooks (Free Algebra Ebooks, Free Geometry Ebooks, Free Calculus Ebooks) that I have listed before, Ebooks Directory offer 500 mathematics ebooks. I did not have time to verify if all of these ebooks are free, but I suppose that most of them are.
You can browse the category page here.
Below is a collection of free algebra ebooks that are downloadable. They include math ebooks on High School Algebra, Trigonometry, College Algebra, Linear Algebra, and Abstract Algebra.
Elementary Algebra and Trigonometry Ebooks
- Advanced Algebra II: Conceptual Explanations by Kenny Felder
- Advanced Algebra II: Activities and Homework by Kenny Felder
- Algebra I by CK-12 Foundation
- CK-12 Trigonometry by Mara Landers and Brenda Meery
- College Algebra by Carl Stitz and Jeff Zeager
- Concepts of Algebra by Christopher Cooper
- Elementary Mathematics by William Chen and Xuan Duong
- Elementary Algebra by Christopher Cooper
- Elementary Algebra by Denny Burzynski and Wade Ellis
- Elementary Trigonometry by W. E. Paterson
- Intermediate Algebra by Department of Mathematics, College of the Redwoods
- Intermediate Algebra by John Blakely
- Trigonometric Delights by Eli Maor
- Trigonometry by Michael Corral
Abstract and Linear Algebra
- A First Course in Linear Algebra, by Rob Beezer.
- A Second Semester of Linear Algebra by S. E. Payne
- Abstract Algebra by John A. Beachy, William D. Blair
- Abstract Algebra On Line by John A. Beachy, William D. Blair
- Abstract Algebra: Theory and Applications by Thomas Judson
- Abstract Algebra:The Basic Graduate Year by Professor Robert Ash
- Algebra: Abstract and Concrete by Frederick M. Goodman
- Elementary Linear Algebra, by Keith Matthews.
- Elements of Abstract and Linear Algebra by E.H. Connell
- Linear Algebra and Jim Hefferon
- Linear Algebra by Macquarie University
- Linear Algebra and Matrices by Martin Fluch
- Linear Algebra Done Wrong by Sergei Treil
- Linear Algebra by Professor Jim Hefferon
- Linear Algebra for Informatics and José Figueroa-O’Farrill
- Linear Algebra: An Introduction to Linear Algebra for Pre-Calculus Students by Tamara A. Carter, Richard A. Tapia, Anne Papakonstantinou
Other Algebra Books
- A Course in Universal Algebra, by Stanley Burris
- Lie Algebras by Professor Sternberg
You may also want to check my Free Calculus Ebooks post.