The Court of the Dead by Rick Riordan and Mark Oshiro

The Court of the Dead is the second book in the Nico di Angelo Adventures, set in the world of Percy Jackson and written by Rick Riordan and Mark Oshiro. It follows on from The Sun and the Star, so start there if you are new to Nico and Will's story.

Three months after coming back from Tartarus, Nico di Angelo and his boyfriend Will Solace travel to Camp Jupiter to visit Nico's sister, Hazel Levesque. Monsters from the Underworld have taken shelter at the camp, and then they start disappearing, one by one. Nico and Will have to face a dark force set on punishing every monster for the past.

Middle-grade fantasy for readers around 10 and up, and for anyone working their way through the Camp Half-Blood books.