In your books you talk about the reincarnation beliefs held by your Keltic characters; is this something you believe in for yourself?
Absolutely. Reincarnation was one of the most basic Druidic tenets; though with them it seems to have been transmigration as well -- you could regress to animal form if you messed up as a human (Gwydion and Gilfaethwy, for instance, in the Mabinogion). But according to the teachings I follow, once you're human you stay human.
But you do reincarnate: When you come to the end of one mortal existence, you die, and then, in the presence of Deity, you yourself -- your soul, you as immortal entity -- stand in judgment on yourself. It's a judgment, but it's not a Final Judgment by any means. And God does not judge you; YOU judge you, in God's presence, and you will see your deeds for what they were, the good motives as well as the bad, and you won't be able to hide or to continue to hornswoggle yourself with rationalizations. You will force yourself to face what you have done.
But. Dig this: You will judge yourself by the HIGHEST thing you have ever done in full intent and knowledge, not the lowest or the worst. The best and finest thing you have done, the highest thought you have entertained, or shown yourself capable of, that is the benchmark to determine future karma. But it must be a real highness -- you can't go around, say, killing and stealing and laying waste to the landscape, and then piously fake a good thought or noble deed or charitable impulse and expect that to determine. What determines is the grain of the soul, the true nature, the deepest inner motive and allegiance to the Light or the Dark.
And the truth can't be fooled. But neither can it be blamed.