The Princess, the Dragon, and the Frog Prince

This is a collection of connected stories: 
* The Birthday Gift, by Elisabeth Waters 
* Dragon in Distress, by Mercedes Lackey & Elisabeth Waters 
* The Dragon's Horde, by Elisabeth Waters & Raul S. Reyes 
* A Prince Among Frogs, by Elisabeth Waters 

The story arc started when Princess Rowena's sorceress aunt gave her a fourteenth-birthday gift that could truly be described as enchanting, which impelled Rowena to leave home and take refuge with a dragon. Then things really spiraled out of control, dragging in Tarma and Kethry, a pregnant mercenary (and being carried off by a dragon while suffering from morning sickness is not a fun experience), and a rather clueless prince. And after tangling with Rowena, the prince needed a witch to rescue him.