My sources of inspiration come from many angles these days. From music to games, and from movies to everyday events, there's always something that sparks my muse in some shape, form, or fashion. The Prince's Angel came about when Shayne and I put two of our own original characters together and wrote two "fanfic" stories using them. When we read each other's story, we knew immediately we had something big.
Those two stories combined her Gabriel, the vampire prince of San Diego from her Blood series books, and one of my older characters named Mychael. (Yes, that's where I got my pen name from, if you're wondering.) Eventually, Gabriel became Mael Black, and Mychael became Cian Carmichael. (To further complicate things, that's how we cemented our pen names: Mychael Black and Shayne Carmichael. *chuckles*)
The Prince's Angel was written in a month--all 90k words of it. Since then, the entire Romanorum world has exploded and we've written what seems like a gazillion stories set within the world. The Romanorum is, in itself, a huge inspiration for both of us. While we've officially finished Mael and Cian's third and final book, I promise you there is much more to come from Diocourides' intricate world of vampires. :)