If you are interested in number theory, you can find the book Number Theory, by S. G. Telang very helpful.
The book can be helpful for everyone. It’s topic are ordered - from basic to advanced.
If you want to practice number theory problem, you can also try this book: 104 Number Theory Problems, by Titu Andreescu, Dorin Andrica, Zuming Feng.
You can find the hardcopy of the books if you contact with Bangladesh Math Olympiad community.