I am looking for book recommendations that will teach me the art of problem solving.
Learning theory is one thing but doing problems in limited time in a test is another.
To increase these skills I am looking for a book that has hard problems of high school level (i.e. pre college level) or maybe undergrad level which will teach me how I should approach a problem / solve it.{From Algebra, Calculus, Coordinate Geometry mainly}[Any books which give extra knowledge of some well known/important theorems , that are a bit above highschool level but not too beyond and can help in problem solving will also be really helpful :)]