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.