Studying calculus from multiple textbooks can enhance understanding by providing diverse perspectives and problem types. Using two books is often beneficial, especially if they cover different approaches or problem styles, while three may lead to confusion or exhaustion. It's generally recommended to focus on the primary textbook used in class, as it aligns with the professor's teaching style, and use supplementary books for additional exercises and concepts. Different books can clarify concepts and offer unique problems, which aids in grasping the material more thoroughly. However, when books differ significantly in organization, such as the order of topics, it's important to adapt your study strategy accordingly, possibly by reconciling the differences or choosing one book as the primary guide.