Book four of the Septimus Heap series. Earlier ones reviewed here, here, and here. Spoilers ahead for them, though not for Queste.
It opens with a prologue, this one about Nicko and Snorri, still back in time, and their last meeting with Marcelleus Pye.
Forward to our time, and we have Merrin plotting revenge against Septimus for stealing his name and place, and Jenna and Septimus planning a rescue for Nicko and Snorri, and Marcia trying to get Septimus to attend to his studies, and Septimus trying to hide from her that he made the potion for Marcellus.
And Merrin gets himself hired as a scribe, and Septimus's friend Beetle fired, but his plans to Darke Septimus's fate don't go well. And Simon sets after him to retrieve Sleuth from him.
This all collides with a ghost named Tertius Fume, and a nasty, malevolent fellow he is, which sets everyone's plans awry. The rest of the story involves a wintry landscape, a trip to the Forest and witches who want a princess, a place where all times are one, the relocation of Septimus's dragon Spit Fyre, Marcia's aggrieved discovery that Simon went and reformed on her just when she wanted a Dark wizard, helping a friend who happens to be much of a rat, and a talk over tea between an Extraordinary Wizard and an Alchemist.
And a quest. Did I mention a quest? For one thing that turns out to be about another.