Library Genesis
Disquisitiones Arithmeticae
Author(s): Carl F. Gauss, W.C. Waterhouse, Arthur A. Clarke, J. Brinkhuis, C. Greiter
Publisher: Springer, Year: 1986
Library Genesis
An Introduction to the Theory of Numbers, Sixth Edition
Author(s): G. H. Hardy, Edward M. Wright; Editors: D. R. Heath-Brown, Joseph H. Silverman
Publisher: Oxford University Press, Year: 2008
Library Genesis
A friendly introduction to number theory
Author(s): Joseph H Silverman
Series: Featured Titles for Number Theory
Publisher: Pearson, Year: 2013
Library Genesis
Problems in geometry
Author(s): Marcel Berger, P. Pansu, J.-P. Berry, X. Saint-Raymond, Silvio Levy
Series: Problem books in mathematics
Problem solving and proofs at the Olympiad level are an entirely different skill from the AMC and AIME competitions.
It is assumed you've completed the Art of Problem Solving Volume 1 and at least some of Volume 2.
There are a number of books both classical and modern that cover non-routine problem solving at the Olympiad level.
The classical resources on problem solving are mostly by the famous mathematician George Polya.
General advanced problem collections are a good place to start, covering a range of topics.
They will also help you with your AIME performance, necessary for Olympiad qualification.
Of course, your main focus should be to practice completely the past problems of the Olympiad you are preparing for, USAJMO, USAMO, etc.
It is assumed you've completed the Art of Problem Solving Volume 1 and most of Volume 2.
The famous general collections from Russia, Poland, and Hungary should be well studied.
I find the Polish book to still be the most relevant for really learning Olympiad level proofs..
4. Hungarian Problem Book I (1894 - 1905) - Rapaport (MAA,1963)
5. Hungarian Problem Book II (1906 - 1928) - Rapaport (MAA,1963)
6. Hungarian Problem Book III (1929 - 1943) - Andy Liu (MAA,2001)
7. Hungarian Problem Book IV (1894 - 1905) - Barrington, Liu, (MAA,2011)
Geometry: Plane Geometry
It is assumed you've completed the Art of Problem Solving Introduction to Geometry.
In my view, the classical plane geometry resources are still the superior choices for study, even though they are very dense.
Start with #1, #2, and #3 (Challenging Problems and Geometry Revisited), however, to do well on the Olympiad, you will need to study the more advanced Altshiller-Court, Johnson, and Aref.
Altshiller-Court and Johnson are very light on problems, Aref is heavy on problems, so they all work together.
Everything you need for Olympiad plane geometry success is right here.
Problem-Solving and Selected Topics in Euclidean Geometry In the Spirit of the Mathematical Olympiads by Louridas, Rassias (2013)
Algebra: Equations and Trigonometry:
It is assumed that you've completed and understand both Art of Problem Solving Introduction to Algebra and Art of Problem Solving Intermediate Algebra.
In my view, the classical Algebra problem books are still the superior choices for study.
The modern resources are far superior choices for study than the older books as they are oriented towards Olympiad competition study.
Start with the tutorials and then the modern books and then if your really inspired take a look at the classical and other books, everything you will need is in the tutorials and modern books.
The classical resources include large amounts of material that is not relevant for high school olympiad contests and though interesting, can eat up your time.
There are no classical books and resources on olympiad functional equations problems.It was all hit or miss back then from various magazine problem sections.
Start with the tutorials, then on to the modern books, then it's just a matter of doing problems. Treat each one as a puzzle.
Discrete Mathematics (Combinatorics, Probability, and Graph Theory):
It is assumed you've finished the Art of Problem Solving Counting and Probability book. The modern treatments are far superior to the classical resources.
There are a number of good textbooks for background, but most include too much as they are oriented towards college courses.
The idea is to pick one and learn it well. I always liked the Tucker book, now in a 6th edition.
The Tucker and Vilenkin books have great coverage of generating functions.
The Art of Problem Solving Intermediate Counting is good also.
It is assumed that you've covered the matieral in the Art of Problem Solving Introduction to Number Theory. The necessary background for Olympiad level number theory can be found in any of dozens of books available that are usually titled "Elementary Number Theory" or some variation.
The idea is to pick one and learn it well. Generally they don't cover diophantine equations that well, which is where the Olympiad problem books come in. The Sierpinski book is the best. Note that at the international olympiad level, you now must also know quadratic reciprocity.
The ones I like are by Roberts, LeVeque, and Dudley. The Roberts book is very unusual for style. Once you know the basics it really is all about doing problems.
An elementary treatise on the theory of equations: with a collection of examples. [3 ed.]
Benjamin Peirce
An elementary Treatise on Plane and Solid Geometry
Charles Smith
An Elementary Treatise on Solid Geometry [11th ed.]
Percival Frost
An Elementary Treatise on Curve Tracing [5th ed.]
A. B. Basset
An elementary treatise on cubic and quartic curves [CUP ed.]
Robert Lachlan
An Elementary Treatise on Modern Pure Geometry
W. J. Johnston
An Elementary Treatise on Analytical Geometry, with Numerous Examples
Grund, Francis Joseph
An elementary treatise on geometry : simplified for beginners not versed in algebra. Part I, containing plane geometry, with its application to the solution of problems [3rd ed., stereotyped.]
Joseph Edwards
An Elementary Treatise on the Differential Calculus: With Applications and Numerous Examples
Charles L. Dodgson
An elementary treatise on determinants
William Woolsey Johnson; John Minot Rice
An elementary treatise on the differential calculus founded on the method of rates or fluxions. [3d rev. ed.]
Henry Thomas Herbert Piaggio
An elementary treatise on differential equations and their applications
Arthur Cayley
An elementary treatise on elliptic functions [2nd ed.]
Thomas Murray MacRobert
An elementary treatise on harmonic functions, with applications [2nd ed.]
William Elwood Byerly
An elementary treatise on Fourier's series and spherical, cylindric, and ellipsoidal harmonics: With Applications to Problems in Mathematical Physics
Jellett J.H.
An elementary treatise on calculus of variations (1850)
Isaac Todhunter
An Elementary Treatise On Laplace's Functions, Lame's Functions and Bessel's Functions [Elibron Classics ed.]
John Minot Rice, William Woolsey Johnson
An Elementary Treatise on the Differential Calculus Founded on the Method of Rates or Fluxions
N. M. Ferrers
An elementary treatise on spherical harmonics and subjects connected with them
George Biddell Airy, K.C.B., M.A., LL.D., D.C.L.
An elementary treatise on partial differential equations. Designed for the use of students in the university (2nd edition, 1873) [2nd ed.]
Tait P.G.
An elementary treatise on quaternions [3ed.]
H. Piaggio
An Elementary Treatise on Diff. Eqns. and Their Applns.