There is a basic book on the subject:
"Basic ESD and I/O Design by Dabral and Maloney (Wiley Interscience ISBN 0-471-25359-6, 1998)
I have a copy but it's a bit out of date.
Suggestion - put together some simulation models for machine and HBM sources, and also model some I/O cells and their diode structures. Looking at what happens during the transient simulations is informative.
Make sure you include parasitic resistances and real diode models (not ideal) and you can learn a lot from that.
http://electronicdesign.com/power/protect-your-fortress-esdGive that a read too - written from the perspective of PCB level design but still useful. I wrote that not too far back.