Alternative journal entry:
The Book of Plenix was recovered from the volus homeworld, Irune. Find someone on the Citadel who can use it.
Listen to the volus standing next to the Elkoss Combine Arsenal Supplies kiosk to the left of the bank podium in the Presidium Commons.
If, for some reason, you never overheard the volus, you can pick up the book and the mission's journal entry during a Search and Rescue operation. Its alternative journal label is Citadel: Book of Plenix.
Scan the volus homeworld to acquire the book regardless of mission acquisition method. It is in the Aru system of the Aethon Cluster. Note: the Aethon Cluster will not appear on the Galaxy Map until after completion of Priority: Tuchanka. Intervening events at the Citadel will delay completion of this mission if you decide to get the book early.