These are well-researched books available for download. More books will be added soon. I do not own all of these materials. I just found it useful and would like to help other for them to have a material for learning.
Mathcounts Handbook .
Schaum’s Easy Outlines in Abstract Algebra
Calculus, Early Transcendentals 6th ed
Number Theory Problems for IMO
Probability and Statistics for Engineering and Sciences
Probability and Combinations Practice Problems For Beginners
Philippine Mathematical Olympiad Problem Compilations
Philippine Statistics Quiz (PSQ) Reviewer
MTAP Reviewers for 4th Year By Russelle Guadalupe