iBooks (Sorry for the lack of link here; if you have an Apple product (phone, iPad, Mac) simply search for my name in iBooks. Brigitte's Cross should be available there today; Blood Sisters is already active).
Smashwords (this site has all formats, including those that would work with Nook).
I had a bad experience early on with PubIt, Nook's early platform for their readers; as a result, I've chosen not to publish directly there, but simply point Nook readers to the Smashwords store.
Remember, you can always order print copies from Amazon (which also still carries the Kindle version) and from CreateSpace, if you're a member (CreateSpace is owned by Amazon, so you're just as well off buying on Amazon.com and getting free shipping).