The Ioun Codex is a 32-page, full-color sourcebook exploring ioun stones, from origins to NPC experts, common and rare uses, monsters and locations, as well as rules for creating your own ioun stones from a selection of over 50 powers.
An excellent sourcebook for anyone who loves ioun stones (and who doesn’t love ioun stones?)
New options, variant ioun stones, and new NPCs make it easy to drop the Codex into any campaign (which is exactly what I did with my lunchtime science fantasy D&D campaign)