My first guide! Woot! This guide shows you where all the Zodiac Tomes' locations throughout Celestia are! The quest is give by Dierdre Parker in Celestia Base Camp, next to the platform that Zeke and the mini-games are.
Survey Camp - With Optio Agenor
The Grotto - With Governor Neverus
District of the Stars - With Brutus Starkwave
The Stellarium - Complete instance
Floating Land - With Bristleclaw
StormRiven - Karalok Nightfinder(?)
StormRiven Hall (this requires going into a mini-instance) - Complete instance
The Portico (otherwise known as the Lunarium) Finish half the dungeon to gain entry to this.
Science Centre - With Muglulp
Crustacean Empire - With Queen Calypso
The Solar District (Test of the Sun) - Halfway through instance.
Trial of the Spheres (final instance) - With Ptolemos