I have really enjoyed the Septimus Heap series. Septimus is the seventh son of a seventh son in a line of wizards. In Magyck, the first in the series, Septimus is stolen the night he is born by a midwife who pronounces him dead. That same night the boy's father comes across a bundle in the snow containing a new born baby girl. The following books reveal who this baby girl is and what happens to Septimus and, eventually, the adventures they share together.
As with the five earlier books, there are sinister forces at work in Darke. One of the characters has been Banished and Septimus must enter the Darke in hopes of retrieving him. The Darke Domaine finds a way to enter the Castle and eventually engulfs it. And a threatening Darke dragon is on the loose.
The covers of the books in this series are fun and creative, too - each cover looks like a locked book. This picture shows the books in the correct order. I haven't read them in order and I feel like I've been able to follow along quite well, but it would probably be best to read them order.
Warner Bros. is working on a movie of the first book which I look forward to with great anticipation.