Magic/Fantasy Book

I read the first book in a series a while back, and now I can't remember anything to figure out what the next book is. It is driving me crazy.

The book was about people who can do magic (can't remember if they were called magicians, mages, warlocks, etc). The thing that sets it apart is that the magic is fueled by that person's life force- doing magic makes you die sooner. Some people learned that they could take that power for a random other person's life force, and up until the series starts, these people have all been men.

Now a girl learns how to do it (with the help of a mentor). They never know who the person is that they are stealing the life from. In this case, it is the prince of the kingdom.

If you've read the book, that should be enough to set it apart from other books. It has to be a relatively well known book since I was able to find it.