An Introduction to Statistical Learning: With Applications in R, Gareth James, Daniela Witten, Trevor Hastie, Rob Tibshirani, 2021 (Springer) - Provides a foundational statistical treatment of linear regression, explaining the least squares method for parameter estimation.
CS229 Lecture Notes: Linear Regression and Gradient Descent, Andrew Ng, Tengyu Ma, 2023Stanford University CS229 Course Notes (Stanford University) - Explains the theoretical and algorithmic methods behind linear regression, including ordinary least squares and gradient descent.