On The Effectiveness of Address Space Layout Randomization is a fantastic introduction to the concepts of virtual versus physical memory, process layout, Write XOR Execute, and the basic idea of a return oriented programming (ROP) attack as well as why it bypasses Write XOR Execute. Finally, ASLR is covered in detail as the authors explain why it is circumventable in a 32 bit architecture.

 
Hovav Shacham, Matthew Page, Ben Pfaff, Eu-Jin Goh, Nagendra Modadugu, and Dan Boneh. 2004. On the effectiveness of address-space randomization. In Proceedings of the 11th ACM conference on Computer and communications security (CCS ’04). ACM, New York, NY, USA, 298-307. DOI=10.1145/1030083.1030124 http://doi.acm.org/10.1145/1030083.1030124