A note to thrifty gamers out there: Icons is on sale at DTRPG as a pay-what-you-want special.
I assume this is because of the announcement of the Assembled Edition coming this summer.
EDIT: Update from the man himself here.
Regardless, if you're interested, it's a good opportunity to take a look at the game for a low price. Also, the Villainomicon is set at the same price as well:
I don;t know that I would call it "essential" to the game but it does have some Q&A, introduces the "pyramid test" concept, and has lots of pre-made bad-guys ready to drop in as needed.