Sheltered by Jen Colly is book Four in "The Cities Below" series. This is the story of Bette Dautry and Rollin Casteel.
Both Bette and Rolling have secrets that they at first try to keep from the other. But they are both drawn to each other so much that Rollin isn't willing to let her go. Bette is even rethinking what she though she would get from Rollin.
I really enjoyed their story and the books I have read in this series. I do think you can make this a standalone book if you choose to do so.