I got it and I am currently running the trilogy.
It has neat stuff on both yig and the yellow sign but not much that I think will directly impact running the trilogy.
The Yig Cosmology from the Pirate's Guide world section is expanded a little and ties into two of the other cults (yellow sign and a norse loviatar one).
The Yellow Sign stuff has more on their history in Freeport and dealings with Drac which would be good t have when running the latter parts of the trilogy.
The Wizard's Guild stuff deals with secrets of the inner ruling council of them and IMO has a neat twist on why the cult is there, one interpretation is that the rituals and evil cult center are symbolic metaphors in an arcane path to power and so not truly evil. This would be the most relevant if your group has a wizard who wants to join the guild.
There is a bunch of info that is from the post trilogy time-frame of the Pirate's Guide, major NPCs who are changed by events in Crisis in Freeport, how the Yellow sign was hunted down after the trilogy, etc.
I am enjoying it immensely, I just wish the promised d20 web enhancement for it would come out soon.
Downsides, I don't like the cannibal cult and the murderous pirate god is so-so IMO.