This volume in Rick Riordan's "Heroes of Olympus" series picks up with Percy and Annabeth in Tartarus, their friends on board the Argo II trying to reach the House of Hades and meet them at the Doors of Death, and Gaea with all her minions still bent on destroying the world. Some characters from previous adventures show up - Khione (the Greek goddess of snow), Calypso (goddess/nymph on the island of Ogygia), Kelli (empousa, vampire cheerleader), Arion (Hazel's magical steed), the Titan Iapetus a.k.a Bob, etc. Many of the monsters they have defeated before have regenerated and are back with a vengeance. As they battle their way through the Mediterranean, their friends back home prepare for war. Octavian, the Roman augur, pushes for an attack on Camp Half-Blood, but Reyna holds the soldiers of Camp Jupiter back from that final confrontation. We see only bits and pieces of the action on the home front through the dreams of those on the quest to reclaim the Athena Parthenos; here a quick view of Grover and Rachel Dare delivering a message to Reyna, there an image of Chiron dressed for war, but they never have enough information to know exactly what's going on - just enough to keep them worried and pushing on with their mission.
One of the strengths of the stories is that Percy and his friends grow and change with each new situation or adversary they overcome. They are challenged by circumstances, their godly parents, and their own desires so that they master their innate powers, learn new skills, and gain confidence and maturity as they push forward. Their enemies always underestimate their courage and determination - so obviously the bad guys don't seem to learn much from their mistakes. Maybe that's a drawback to being eternal creatures that get regenerated time and time again - maybe they come back the same every time, with no wisdom gained from all their years of existence.
If you've been following Percy since The Lightning Thief, then you will enjoy this installment and be looking forward eagerly to the final book in the Heroes of Olympus and rooting for your favorite heroes. While you are waiting to see how it all ends, you can enjoy visiting the websites for the books or the author, or watch the trailer. I just added this book to the library to celebrate my birthday and there is already a waiting list to check it out.