I read a book, over ten years ago now, and if I recall it seemed like it was maybe the second or third book in the series. It was about a man who had either been pulled into a book, or the past. (I'm not 100% sure as to which it was) and once there he was made lord of the Castle and could transform into a dragon.
It seems that he might also have been able to use other forms of magic, and that his wife or girlfriend (again, I am fuzzy on the details) was also with him. I really enjoyed the book and now I would like to see if there was more to the series, and I would really appreciate it if someone could tell me what it was I read.