For code reviews, go here: http://fabiensanglard.net/
A small but complete open source rendering engine that has easy to read code. I don't think that can exist. A production engine will be hard to read. But if you find one, please post!
I would however, recommend these books:
Physically Based Rendering, Second Edition: From Theory To Implementation
by Matt Pharr et al.
Game Engine Architecture, Second Edition
by Jason Gregory
Thanks! I have ordered for now the Game Engine architecture book and have put in my list the PBR one
Do you know some open source (preferably using OpenGL) rendering engines that are reasonably small (to read the code easily), but good enough to take as example to start with?
Check out Horde3D (OpenGL). It's a small and easy to read open source rendering engine (supports both forward and deferred rendering and uses a (limited) data-driven architecture). So it should be a good starting point.
I'm also releasing my small open source engine in a couple of weeks
Thank you! I'll have a closer look to Horde. Can't wait to see yours as well!