Free Online Textbooks: from Pre-Calculus to Number Theory
Pre-Calculus “OpenStax Text”: https://d3bxy9euw4e147.cloudfront.net/oscms-qa/media/documents/Precalculus-LR.pdf
College Algebra by Carl Stitz and Jeff Zeager: http://stitz-zeager.com/szca07042013.pdf
Basic Calculus Refresher by Ismor Fischer: http://www.stat.wisc.edu/~ifischer/calculus.pdf
Apex Calculus by Gregory Hartman: https://drive.google.com/file/d/0B2Pdg3LqVPNPcFlGTnR4a3NuMmM/view?pref=2&pli=1
Single Variable Calculus by David Guichard: https://www.whitman.edu/mathematics/calculus/calculus.pdf
Applied Discrete Structures by Al Doerr and Ken Levasseur: http://faculty.uml.edu/klevasseur/ads/
Elements of Discrete Mathematics by Richard Hammack https://www.people.vcu.edu/~rhammack/Discrete/Alpha.pdf
Elementary Differential Equations by William F. Trench : http://ramanujan.math.trinity.edu/wtrench/texts/TRENCH_FREE_DIFFEQ_I.PDF
Linear Algebra by Jim Hefferon: http://joshua.smcvt.edu/linearalgebra/book.pdf
Mathematical Reasoning “Book of Proof” by Richard Hammack: http://www.people.vcu.edu/~rhammack/BookOfProof/Main.pdf
Abstract Algebra Theory and Applications by Thomas Judson: http://abstract.ups.edu/aata/aata.html
Real Analysis by Brian Thomson and Andrew Bruckner: http://classicalrealanalysis.info/documents/TBB-AllChapters-Landscape.pdf
Elementary Number Theory by Wissam Raji: https://www.saylor.org/site/wp-content/uploads/2013/05/An-Introductory-in-Elementary-Number-Theory.pdf