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 .

Brittish Math Olympiad

Schaum’s Easy Outlines in Abstract Algebra

Best Number Theory Book

Calculus, Early Transcendentals 6th ed

History of Mathematics

Number Theory Problems for IMO

USC High School Math Contest 

Probability and Statistics for Engineering and Sciences

Combinatorial Probability

Probability and Combinations Practice Problems For Beginners

Algebra and Trigonometry

Fundamentals of Geometry

Philippine Mathematical Olympiad Problem Compilations

Philippine Statistics Quiz (PSQ) Reviewer

MTAP Reviewers for 4th Year By Russelle Guadalupe

Reviewer 1

Reviewer 2

Reviewer 3

Reviewer 4

Reviewer 5

Reviewer 6

Answer Keys to 5 sets of Reviewer

MTAP reviewer for grade 3