Book number 2 in the Heroes of Olympus series by Rick Riordan.

This book sees the return of Percy Jackson but, as with Jason Grace, Percy has lost his memory. Percy finds himself in Camp Jupiter, after carrying the Goddess Juno over the river Tiber into camp. He saves, and is helped by, Frank and Hazel both of whom belong to the fifth cohort. Hazel a daughter of Pluto (Hades in Greece) and Frank we find out is a son of Mars (Ares). It is proclaimed by Mars that Frank and Percy shall go on a quest to Alaska to kill a giant and free Thanatos (Thanatos is the personification of death, a bit like DEATH in The Disc-world) guardian of the gates of Death. Frank chooses Hazel to accompany them.

Their journey is fraught with monsters and choices that will change their lives irrevocably. Along the way they pick up a Harpy called Ella and Percy starts regaining his memories.

Another good read, I look forward to starting book 3 later.