IPhO

The International Physics Olympiad (IPhO) is the World Championship Physics Competition for High School students and is held annually in a different country. The first IPhO was held in 1967 in Poland. It has gradually expanded to over 80 countries from 5 continents. The IPhO International Board ensures that the competition takes place each year and that each host country observes the regulations and traditions of the IPhO.
The IPHO requires a really deep theoretical knowledge of physics and application also in the problems:-
This is the offical syllabus it is of 2018 but you can check it for 2019 but its preety much the same- 
https://ipho2018.pt/content/syllabus
so the topics are in this but I assume that there may be many more in the books I recommend.
There is a long list of books and unfortunately I dont have the pdf to them but if you have please share the link with everyone this blog...

General Physics books:

1. Physics – Resnick, Halliday and Krane, volumes I and II. 
2. Fundamentals of Physics – Resnick, Halliday and Walker.
3.  Fundamental Laws of Mechanics – I. E. Irodov.
4. Basic Laws of Electromagnetism – I. E. Irodov. (**)
5. Physics – Alonso and Finn. 
6. University Physics – Sears & Zemansky. 

General Problem books:

1. Problems in General Physics – I. E. Irodov. 
2. Aptitude test problems in Physics – S. S. Krotov. 
3. 200 Puzzling Problems – Gyula, Riley and Honyek. 
4. Problems in Elementary Physics – Bukhovtsev et al. 

General Physics:

1. Feynman lectures in Physics, Vol I and II – Feynman, Leighton and Sands. 
2. Berkeley Physics series, Vol I through V.  

Mechanics:

1. Mechanics – Kleppner.
2. Classical Mechanics – David Morin.
3. Mechanics – Landau and Lifshitz. 

Electromagnetism:

1. Introduction to Electrodynamics – D. J. Griffiths. 
2. Foundations of Electromagnetic Theory – Reitz, Milford, Christy. ~ 
3. Electricity and Magnetism – E. M. Purcell,
4. Electromagnetic Fields and Waves by Paul Lorrain and Dale Corson

Thermodynamics:

1. Heat and Thermodynamics – Zemansky and Dittman.
2. Thermodynamics – Herbert Callen.
3. Feynman lectures, Vol I – relevant chapters for thermodynamics.
4. Statistical Physics, part I – Landau and Lifshitz. 

Optics and Waves:

1. Optics – Hecht 

Relativity:

1. Introduction to Special Relativity – Robert Resnick. 
2. Space and Time in Special Relativity – N. D. Mermin.
3. Spacetime Physics – Taylor and Wheeler

Others:

College Physics by Alan Giambattista, Betty McCarthy Richardson, and Robert C.Richardson

Physics by John D. Cutnell, Kenneth W. Johnson
Study first year level university physics.
University Physics with Modern Physics with Mastering Physics , by Hugh D. Young and Roger A. Freedman
Physics for Scientists and Engineers with Modern Physics by Raymond A. Serway and John W. Jewett, Jr.
Classical Mechanics by John R. Taylor

Classical Dynamics of Particles and Systems by Jerry B. Marion and Stephen T. Thornton
Special Relativity by A.P. French
Vibrations and Waves by A.P. French

Selected problems from "A Guide to Physics Problems", Part 1 and Part 2 by Sidney B. Cahn and Boris E. Nadgorny

Practice is the key to an olympiad like this so practice from various national Olympiads even not from your country and international olympiads like Asian Pacific Physics Olympiad,etc.

All the best!