The third Septimus Heap book. The first two reviewed here and here. May have spoilers for them, but not for Physik.
It opens with a prologue in which Silas Heap and Gringe open a Sealed room in the castle, to store something in. It has a portrait, which Silas dumps out without considering why anyone would Seal up a room with a portrait.
So we start the story with Snorri arriving in her boat to trade, though she's only fourteen -- she hopes to find her father's ghost. And she finds a Port with a disease traveling about, and the Rat Catchers gathering to kill rats, which they blame.
Meanwhille, Septimus, who ought to be studying for a test, instead studies Physik in hopes of finding a cure. Jenna is haunted by the ghost of Queen Ethelredda the Awful, and because of it, ends up losing Septimus though a glass that carries him into the past. He is captured there and made an Apprentice. And Jenna's frantic search for him has its own consequences.
It weaves on through ooddles of gold, baby princesses haunting the castle behind a wainscot, a discovery of why several ghosts call Jenna, a diary, a man whose heart beats every five minutes, a page boy, a pink paddle boat, and an intricate plot through time.