My son and I have been reading Diana Wynne Jones together, both on audio and me reading aloud to him.  This is the second book in the duology that begins with Deep Secret, which so far only I have read.

The Merlin Conspiracy by Diana Wynne Jones. Collins (UK) and Greenwillow Books (US), 2003.
Nick, a secondary character from Deep Secret, lives in our London, wishing he could find a way to travel to the other worlds he knows are out there. He is at a convention with his dad, who hopes to meet one of his favorite authors.  (Diversity note: Nick’s dad is adoptive, and Nick himself, while from another world and raised in England, looks South Asian.) One step later and Nick finds himself in another world, boarding a strange flying machine as part of the magical security team for that very different England’s prince.  It takes him a while to realize that he’s really there, long enough to create some serious trouble, like running afoul of the powerful priests of a third world and  befriending a world-wandering elephant name Mini.

Meanwhile, Roddy (short for Arianrhod) lives in the alternate England called Blest, where the magic is kept both strong and controlled by a complex bureaucracy.  Roddy’s parents are both wizards in the King’s Court, so that Roddy travels by bus around the kingdom with her parents, the King, the Merlin, and the rest of the Court.  Trouble arises as her best friend Grundo’s mother Sybil, never well-meaning, begins a plot to take over the kingdom and subvert its magic with the help of a new Merlin.  The problem is that none of the adults who might be able to help will believe that anything is going on.  The only exception is her grandfather from Wales, who’s too caught up in the magic of the land himself to do anything but direct Roddy towards acquiring the extensive magical knowledge of a wise woman from centuries earlier.

It takes a while for Nick and Roddy to meet, and even longer for them to figure out what needs to be done to save the worlds.  The path there is filled with vivid worlds and people, with brief visits turning out to be highly significant later down the road.  The group of people accompanying Nick and Roddy gets both more numerous and increasingly rowdy as the story progresses and complete disaster looms ever larger, but only they are able to see and communicate with the deep spirits of the land who are being threatened.

This seems classic Diana Wynne Jones – so very much going on, with characters and places both familiar and improbably vividly imagined.  Alas, her familiar note of fat shaming is included as well – the evil Sybil is described as having fat ankles, and ridiculed for showing them off by going barefoot. We don’t have much time to read together anymore, so it took us five months to get through the 500-page book, but we enjoyed it all the way and kept trying to find more time to read.  We’d started with the second book because I’d already read the first, but the stories are separate enough that this worked just fine.

While I wish there were more than two books in this particular series, for anyone wanting to try Diana Wynne Jones but intimidated by her massive library in general and Chrestomanci in particular (as I was for many years), Deep Secret and The Merlin Conspiracy are a good starting place.  Of course no one can duplicate Diana Wynne Jones’s imagination, but books like Frances Hardinge’s A Face Like Glass and Nnedi Okorafor’s Akata Witch have similarly complex world-building with appeal for kids and adults alike.