For general reading: Jaan Puhvel's Comparative Mythology and Heaven, Heroes, and Happiness by Shan M. M. Winn. Both have helped my thinking about Indo-European polytheisms, and I know I'm going to look again at the Roman chapter in Puhvel's book fairly soon. Could be useful as a start at looking at similarities between Roman and Celtic traditions.
For the website, the Temple of Brigantia's Classical Celtic Wicca page. I know this sounds weird, but their Wiccan rituals are dedicated to Romano-Celtic deities, and I should probably credit their extensive list of deities for nudging me in this direction. Jane Raeburn has written a little book out of their tradition, Celtic Wicca, which has rituals to Sucellos and Rosmerta among others. It's not bad--a big improvement over the standard Llewellyn fare, for certain.