- What is Mathematics, An Elementary Approach to Ideas and Methods - Courant, Robbins, Stewart
- How to Solve It, A New Aspect of Mathematical Solving - Polya
- Introductory Mathematics, Algebra and Analysis - Smith
- A Transition to Advanced Mathematics - Smith, Eggen, St. Andre
- An Introduction to Mathematical Reasoning - Eccles
- A Book of Proof - Hammack
- How to Prove It, A Structured Approach - Velleman
By "Transitional", I mean a book that can take the reader from college Mathematics to the more rigorous and abstract Mathematics taught to Math Majors.
By "college Mathematics", I mean all the scientific majors that include Mathematics in the curriculum, Mathematics Major excluded. Honors Math programs, with their rigour and proofs, quite differ from other Math courses that are generally more problem-solving-based. Although Cambridge and Oxford use these books, I am asking about books used by the US education system - more than the European or any other system in the world.