Irune: Book of Plenix is a side-mission you will be able to start before Priority: Citadel 2; however, to complete it, you must finish the first main mission of the second act
When to start
Before Priority: Perseus Veil
“A volus on the Citadel needs the Book of Plenix to inspire his people.
Recover it from Irune and return it to him in the Presidium Commons”
Citadel Defense Force War Asset
The quest giver is a Volus near the Bank in the Presidium Commons.
To get, the Book of Plenix, you have to use the Galaxy Map and find a cluster named Aethon.
Enter the system, then find Planet Irune and scan it.
Next, enter Irune’s orbit and retrieve the book, then return to the Citadel and find the quest-giver.
NOTE: If you can’t access Aethon Cluster, then you have to play additional main-missions.
Next side-mission is Arrae: Ex-Cerberus Scientists.