In order to (potentially) achieve a medal, mastering calculus-based physics is mandatory. After doing so, completing a few subject-specific textbooks is recommended and of course, completing past IPhO problems is extremely helpful to understand the difficulty and style of questions. You may simply not have enough time to complete all of these textbooks and problem sets (only with a high school physics basis, however, is obviously not enough), so choose which areas you are weaker on or want to improve on and work on those first.
Note: click on the book name to see where you can buy/access these books.
Goes over everything that can be covered in IPhO problems
Check out this unofficial guide on topics that are usually covered in IPhO
Physics by Halliday, Resnick, and Krane (HRK) 5th Edition
Covers a majority of fundamental concepts needed for IPhO, e.g. Mechanics, EM, Thermodynamics, Optics, Fluids, Sound and Waves and Modern Physics
Great book to learn calculus-based physics from
Make sure to do the problems in the book as well
An Introduction to Classical Mechanics by David Morin
Focuses on mechanics at the IPhO, but quite math intensive: need to know calculus very well
Has many classic mechanics Problems, including great relativity chapters as well
An Introduction to Mechanics by Kleppner and Kolenkow
Focuses on Mechanics, similar in difficulty and topics covered to Morin, but not as well-presented (views may vary, though). There is pretty much no point in doing both textbooks.
Electricity and Magnetism (E&M) by Purcell and Morin
Focuses on Electricity and Magnetism, USAPhO(TST)/IPhO level
Optics by Eugene Hecht
Focuses on Optics (leaning more towards IPhO due to matrix methods)
Concepts in Thermal Physics by Stephen J. Blundell, Katherine M. Blundell
A very good and in-depth book on Thermodynamics at the IPhO level, concise and short
Vibrations and Waves by A.P French.
Great revision of concepts and lots of interesting problems
Past IPhO problems and solutions: found here
This is the most precious resource you can have!!
Start from the earlier years (easier) and work your way up; if IPhO problems is too difficult at first, check out past USAPhO problems
More: 200 Puzzling Physics Problems by Gnadig (USAPhO/IPhO level)
Problems in General Physics by I.E Irodov. (no paper version currently available)
Jaan Kalda's Handouts (no paper version currently available). Handouts found here, solutions found here.
Physics to a Degree by Thomas and Raine (USAPhO/IPhO level)