This is book 1 of the Septimus Heap series, which I have read all six of. It leans closer to the Narnia series than Harry Potter or Percy Jackson, because while there are continuing threads and cause-and-effect between the books, there is no overarching threat that unites it all. Somewhat more connectivity than the Narnia books, since they are continuous in time.
This is the one that starts with the take-over. Silas Heap, returning to where his wife Sarah is having their seventh son, finds a newborn baby girl in the snow. And he is intercepted on the way by Marcia, who tells him that they must say she was born to them, and who is inexplicably wearing the amulet of the Extra-Ordinary Wizard. On the arrival home, the Midwife is rushing off with the bandaged body of their son, and Sarah weeps at the sight of the baby, whom they name Jenna.
Only later do they learn that the Queen and the former Extraordinary Wizard had been murdered, the newborn baby Princess had vanished, and that the kingdom had been taken over by the Supreme Custodian.
Ten years later, Alther, being a ghost, learns that they have learned where the princess is. Marcia arrives at the Heaps' to announce it and take Jenna with her while they scatter to escape. On their return to the Wizard's Tower, they find a ten-year-old boy trapped in the Young Army -- Boy 412, as they call him -- and set to guard the Tower, half-dead from cold, and bring him to save him. When Silas and his son Nicko arrive, they are followed and must go down the garbage shoot.
And so the adventures continue. Including a message rat; their Aunt Zelda, the Keeper; too much cabbage; a ring and a boat, both about dragons; an apprentice called Septimus Heap; the return of a necromancer; and a tavern burning down. It all works together to build to a grand resolution.
The world is an interesting mish-mash of modern day and pseudo-medieval -- the Supreme Custodian is far more totalitarian than an Evil Overlord's lieutenant. The magic has some interesting twists. And the tendency toward Capital Letters and the use of bold for magickal words works quite well.