An Introduction to Statistical Learning: With Applications in R, Gareth James, Daniela Witten, Trevor Hastie, Robert Tibshirani, 2021 (Springer) - A widely acclaimed textbook providing an accessible explanation of R-squared as a regression metric, discussing its use, interpretation, and limitations in statistical learning problems.
Applied Linear Statistical Models, Michael H. Kutner, Christopher J. Nachtsheim, John Neter, William Li, 2005 (McGraw Hill) - A comprehensive reference for linear models, this book provides a thorough discussion of R-squared, including its calculation, interpretation, and practical considerations for model assessment.