I have created a very unfinished Introduction to Mesa  its just meant to give you a bit of an overview of how Mesa fits together so you have somewhere to get started. I'm definitely no expert in Mesa or driver development but my other advice would be (if you haven't already) get a good OpenGL book and start reading. Next find a small extension you are interested in and have a go at implementing it. If the extension you pick is an extension that involves glsl you will also need to read up on yacc(bison) as this is what is used to implement the glsl compiler in Mesa.
I just have a question. I just tested my development laptop with a commercial software, and I found a problem in the libdricore 9.2 library. Specifically, it's a polygon issue. The same problem was filed for Blender 3D's VBO and polygon selection for mesa 9.0 and was supposedly fixed. Though the problem is no longer persists for Blender3D, apparently it still is there for other use case.
The problem was the same as what was reported for mesa 9.0, the same file and the same line;
t_vb_program.c, line 369 function CLEAN_4V(machine->VertAttribs[attr], size, data);
How can I check to see if I could debug and get more helpful information about it?