Overhear the angry elcor just outside the Purgatory elevator on the Citadel.
Alternatively, if you never went anywhere near Purgatory after the failed Cerberus coup, you can retrieve the item and the mission's journal entry on doing a Search and Rescue operation. The mission is then labeled as Citadel: Code of the Ancients.
The Code of the Ancients is on the elcor homeworld, in the Phontes system in the Silean Nebula. Note: the Phontes system will be accessible only after completion of the Priority: Rannoch mission. Scan the planet to retrieve the book regardless of mission acquisition method. Give the book to the elcor just outside the elevator to Purgatory.