This campaign could easily cross over with other HREF Earth Setting campaigns, like the HR5 Glory of Rome or HR1 Vikings.
Others have suggested that this book is also ideal to use with Fairy mythology even delving into the Feywild or Fairy Planes.
The Authof or HR3 Celts, Graeme Davis, is a member of The Piazza and has a Q&A thread about his book here.
What source material can be used to expand a Celtic World campaign?
From the Avalanche Press series:
- Celtic Age: Role-Playing the Myths
- Heroes & Monsters of the Celts
- The Little People: A d20 Guide to Fairies,
GURPS CELTS could also be very useful.
How would you set up the campaign? What would be its focus?