If you've been active within the Chemistry Olympiad community, you might have noticed one textbook in particular is mentioned very frequently: Atkins' Chemical Principles: The Quest For Insight.
Atkins is a well-written textbook that introduces all of the necessary fundamentals of Chemistry for the USNCO^[United States National Chemistry Olympiad. Many campers say that Atkins is enough for the non-organic problems in the International Chemistry Olympiad (IChO) exam as well] exam.
During your journey, there is a time when you might question yourself whether this text is enough for Chemistry Olympiads. You might start looking into more advanced books, read them, and then forget about everything you learned the month after.
Don't worry, its a process everyone has had at least once in their journey. If you have these thoughts, keep in mind that:
• The theoretical knowledge from Chemical Principles is enough for 90% of the types of problems in all levels of Chemistry Olympiad. This is most certainly true if you are competing in the United States.
• The remaining 10% that isn't in chemical principles is either
1) organic chemistry, which you should read Klein's Organic Chemistry 1 to learn
2) trivial reactions, which you should just pick up reading online resources and solving problems
3) common advanced topics that show up in chemistry Olympiad (MCB, CFT, spectroscopy, Slater's Rules, etc.)
Most of these can be learned using Wikipedia or ChemGuide. ^[CODS is planning to have guides on 3) specifically, so watch out for new content as they may cover some of these "advanced topics" that show up.]