For those self-studying mathematical thinking at the undergraduate level, several textbooks are highly recommended. "How to Study as a Mathematics Major" by Lara Alcock provides foundational strategies for approaching mathematical concepts. "Book of Proof" by Richard Hammack is frequently suggested for first-year students and is available for free, making it accessible for self-learners, complete with solutions for practice. Another notable text is "How to Think about Analysis," also by Alcock, which revisits calculus from a mathematical perspective. Engaging with these texts is emphasized, particularly through the practice of reading and writing proofs, which is crucial for deepening understanding. Revisiting earlier mathematical topics not only reinforces prior knowledge but also aids in preparing for exams like the GRE. The importance of actively engaging with textbooks and practicing proofs is highlighted as essential for mastering mathematical concepts.