First, I highly recommend reading the Percy Jackson books first (but don't bother with the movie -- it kinda sucked). It's not essential for understanding the story, but that way you're able to jump right in and enjoy.
Lost Hero takes place a few months after Last Olympian. Enter Jason, a boy with no memory, and his friends Piper and Leo. They make their way into Camp Half Blood, and are granted a quest to rescue a goddess. But this isn't simply a rehashing of Percy Jackson, for there are surprising complications, and of course, new monsters to face. We briefly get to see Annabeth, who is fiercely searching for Percy Jackson who is missing. And Rachel makes an appearance, and a prophecy or two. This was a fun read, with great characters, and just different enough that you don't feel like you're being told the same story as last time.
Fair warning: Lost Hero is the first in the series, so by reading this, you are setting yourself up for a long wait. This story wraps up one story arc, but throws out teasers of what's to come.
As an aside, I miss the awesome chapter titles from the Percy Jackson books.
These are also great books to listen to. Jesse Bernstein did a great job of narrating the Percy Jackson books.