The first book of the series Septimus Heap, written by Angie Sage. It is a fantasy book that takes place in our future world, filled with magyk (that's what they call it,) and lots of adventure.

Here's the description of the book, taken from the Septimus Heap website:

The seventh son of the seventh son, aptly named Septimus Heap, is stolen the night he is born by a midwife who pronounces him dead. That same night, the baby's father, Silas Heap, comes across a bundle in the snow containing a new born girl with violet eyes. The Heaps take this helpless newborn into their home, name her Jenna, and raise her as their own. But who is this mysterious baby girl, and what really happened to their beloved son Septimus?

This is the link to the Septimus Heap website.