Heraldo has a post on him.  I’ll post some more stuff later.

Addendum: Cristina also has a post on topic, with the NYT obit link.

Second addendum: I did not know Frank well.  I actually never met him in person.  We had an occasional email exchange, particularly when I posted something about the Abraham Lincoln Brigade or other historial lefty stuff.  He was a wealth of information, and obviously had a passion for the subject and a rich sense of humor.  I knew his artwork from the Mateel.  I wish I had the opportunity to get to know him better.  But I feel richer from the little contact I had with him.  I wish I could say more.