Linear Least Squares. Linear Alg perspective

Hi everyone

I am working on basic topics relating to the Linear Least Squares
method with a Linear Algebra and Numerical Analysis perspective.

I need to read a text that shows the matrices, and the theorems with
proofs.

I need to prove that "min||Ax-b||^2 is a quadratic, semidefinite,
positive" quantity (or problem). Can anybody help me find a webpage or
a textbook on that material?

I appreciate your help and pointers.

.