Not understanding, implementing.

- Derivation of the Normal Equation for linear regression
- Normal Equation and Matrix Calculus
- Wikipedia entry on linear least square
- Matrix calculus Wikipedia page
- it's quite messy but readable.

- Another derivation of the matrix form.

Implementation in C

- A practical description of the simple linear regression algorithm (1-variable) and example.
- Here is one, two more dubious sources (here and here).
- Perhaps the best resource is GSL, which has probably the best reference on linear regression and its implementation here
- A practical description of algorithm and example.

(less-related and equally interesting the non-linear regression description.)

Books:

- Regression by Bingham and Fry
- Matrix Algebra by Abadi and Magnus