I'd recommend Miranda Green, she's written a lot about the 'celts', but her book 'Symbol and Image in Celtic Religious Art' will introduce you to a much broader interpretation of native gods/goddesses...
And for visuality the Gundestrup Cauldron is a good source of material....
http://en.wikipedia.org/wiki/Gundestrup_cauldron#Plate_E:_Warriors_and_Cauldron