[jsnell]

Everything from HOPL II is great. I don't know whether all of it ever got published online, but the book version is totally worth it.

[jcurbo]

It looks like you have to be an ACM member to access these, but all the papers are online for HOPL II: http://dl.acm.org/citation.cfm?id=234286

HOPL III is here: http://dl.acm.org/citation.cfm?id=1238844&coll=DL&dl=GUIDE&C...

HOPL I is here: http://dl.acm.org/citation.cfm?id=800025&coll=DL&dl=GUIDE&CF...

I have the hardcopy versions of I and II that I bought used from Amazon and they're great.