Along Came a Prince by Jennifer Ashley

Sonia Broadbent is astonished to discover that the suitor she believed was the dullest man in creation turns out to be a prince in hiding. Dominik Laszlo Townsend, who took the surname of his foster family, has been incognito in England for years, hiding his true identity from his enemies. The only light in his life is visiting Sonia, where he absorbs her beauty while pretending to be the very tedious Mr. Townsend.
A shocking kiss opens Sonia’s eyes to the vibrant man beneath the facade, but is there anywhere safe in the world where she can fall in love with Prince Dominik?