The Mark Of Athena.

So in the previous book, The Son of Neptune, Percy with a forced amnesia (by Hera, of course), found himself in a new place. He found himself in Camp Jupiter. He was among Roman Demigods and was soon sent on a quest with Hazel Levesque (daughter of Pluto) and Frank Zhang (son of Mars) to help defend themselves from the Earth Goddess, Gaea, and her vicious forces.

In the next book, 'The Mark Of Athena', Percy gets his memory back and gets reunited with his girlfriend, Annabeth Chase.
'The Prophecy of the Seven' quest begins in this book and the chosen seven- Percy Jackson (son of Poseidon), Annabeth Chase (daughter of Athena), Jason Grace (son of Jupiter), Piper McLean (daughter of Aphrodite), Hazel Levesque (daughter of Pluto), Frank Zhang (son of Mars), and Leo Valdez (son of Hephaestus) begin their quest. They have to rescue Hazel's brother, Nico from the Giants, find the Doors of Death, fight a lot of monsters and bring the Romans and Greeks together to defeat Gaea and her minions. Annabeth here, has been assigned with her own personal quest, to find out about the legend of the Mark of Athena and follow where it leads to. A significant part of the story takes place on Argo II, a ship designed by Leo Valdez.
Out of the seven, six of the demigods are in relationships with each other (Percy-Annabeth, Jason-Piper and Frank-Hazel) which gives this story a touch of drama, but I'm thankful it isn't overdone.
Even Tyson, Percy's half-brother and Ella, a red harpy, though an unusual couple are said to be in love with each other though they aren't a significant part of the book.

The story unwinds slowly, leaving the reader in a thirst to know what is going to happen next.
Well, I was reading the book till two hours past midnight and was so anxious to know what happened next. I just couldn't stop. And the end, MY GOD, THE END. I just couldn't stop after that too! I just had to think of what was going to happen next. I guess all the information I've given you is just enough. If you haven't read the book till now, PICK IT UP RIGHT NOW, AND READ IT.

P.S. You may not understand the book unless you've read all the books in the 'Percy Jackson' and 'Heroes of the Olympus' series, so I suggest you read those awesome books too!

Don't have The Mark of Athena?
Buy it on Amazon:
http://www.amazon.com/dp/1423140605