I learned about the projection matrix during a course on mathematical modeling and I'm willing to share some basic concepts:
Concepts
Basic idea (*Coo stands for coordinates, Para for parameter) Camera Matrix Persp Projection Intrinsic Para Matrix World Coo <---> Camera Coo <---> Image Plane Coo <---> Pixel Coo ↑ ↑ └━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┘ Projection Matrix
Pinhole camera model which is the easiest linear camera model we can conceive of.
Transformation/Rotation matrices means translation and rotation of objects based on our view.
Perspective projection has some explanation, and a link to camera matrix.
Notations
Homogenous coordinates which is rather useful. You may see that we can scale the homogenous coordinates without changing the point's coordinate in the projection plane.
Further Materials
Fundamentals of Computer Vision page has slides and notes to computer vision and digital camera related topics. See slide/notes #1.
Geometric Framework for Vision I: Single View and Two-View Geometry by Andrew Zisserman has some fine material on projection matrices.
3D Math Primer for Graphics and Game Development, 2ndE, a decent book on 3D gaming math basics. There are prepared slides and full code in C++.