I only have one book that deals specifically with Celts-

Lords of Battle (The world of the Celtic Warrior)- Written by Stephen Allen and published by Osprey. I have only seen it in Hard-cover and don't know whether soft is available.

The book tracks the history (as far as is possible) of the Celts from the rise and decline of the Halstatt culture through to Boudica's Rebellion and beyond.

Well written and concise.....but only one chapter deals specifically with Vercingetorix and Caesar's campaigns.

Cheers,

Quilts