If you’re new to the Olympiad, take a look at the syllabus to see the covered topics!
Some other resources we recommend are:
- An Introduction to Modern Astrophysics by Bradley Carroll and Dale Ostlie
- Fundamental Astronomy by Karttunen et. al.
- abebooks.com sells used textbooks at a low price and in great condition.
- edX, Coursera, and other online educational platforms often have astronomy or astrophysics related courses.
- Stellarium is one of many resources (but probably the best) for studying constellations and Deep Sky Objects.