The Design of LLVM
In the last five years, LLVM has evolved from an academic project to the universal back-end of C, C++, and Objective C compilers. The key to its success is its performance and adaptability, both of which derive from its unique design and implementation.
To me, this is some of the best work Apple is doing right now.
Looks like this is from The Architecture of Open Source Applications: http://www.aosabook.org/en/llvm.html
There's some other good stuff on the site as well.