From the introduction:
Celts are often a source of inspiration for gamers, in their heroic-fantasy campaigns. And in fact, the bard and druid classes owe primarily to the Celtic lore, before having become archetypes of role-playing games.
[T]his D20 supplement delves into the Celtic mythology, to describe a pantheon and religion with a strong Celtic flair. As such, this netbook owes more to movies and comics than actual history. All the content of this document is intended for game purpose only, and has no historical / literacy value otherwise.