Classical mathematics textbooks essential for advanced and undergraduate mathematicians include Richard Courant's "Differential and Integral Calculus" (volumes 1 and 2) and the two-volume "Methods of Mathematical Physics" by Courant and Hilbert. These texts are highly recommended for their foundational content and affordability, with prices around $15 each for the Courant volumes. Additionally, Segre Lang's extensive series covering various mathematical fields and levels is noted as valuable. Texts by Walter Rudin are also mentioned as significant contributions to the field. The discussion emphasizes the importance of these works for anyone serious about pursuing mathematics.