The Lost Stories is a collection of enjoyable short stories that tie up some loose ends, continue the tale, and provide answers to some interesting but nonessential questions about the series. Most of the stories take place after the events of book 10, The Emperor of Nihon-Ja, but a few also cover incidents prior to the first book and concurrent to the series.
The content is light and includes kissing, violence, killing, and gruesome injuries in the same manor seen in the prior books. If you have read the earlier books, than you should not have a problem. I recommend it to those who have read all of the previous books, but for first time readers - I strongly recommend The Ruins of Gorlan (Ranger's Apprentice, Book 1).