This is the first book of a series, Heroes of Olympus, which picks up not long after the end of the author's series called Percy Jackson & the Olympians. At the writing of this review, the second book in the new series, The Son of Neptune, had just been released (I'll be reading it soon!) 
 Rick Riordan's novels based on Greek and Roman mythology are wonderful! They are full of adventure and surprises, and for anyone familiar with the mythology which is the basis for the stories, every new character you meet becomes a guessing game; because every villain in the books is based on a villain from the old myths. It was fun in this book to get to know a whole new batch of demigods; most of the characters from the previous books are absent, and even the ones that appear have smaller parts to play in this installment. I'd say that this book can stand on its own without requiring you to read the previous novels first, but a trip through the previous pentalogy (The Lightning Thief, The Sea of Monsters, The Titan's Curse, The Battle of the Labyrinth, The Last Olympian) will give you some backstory to help you appreciate what's going on a little better... some characters (Chiron, for example) are not developed in full in this book because the reader is presumably already acquainted with them from the previous series. And of course, you will have no idea how significant the absence of Percy Jackson himself is in this book unless you've read the previous books. But if you choose to start off with this book, the previous series should work well for you as "prequels" to this series, which promises to be even larger in scope than the original series. 
 A year or so ago I enjoyed this story in book form; this time I listened to the audiobook, and I enjoyed it immensely both ways. For me, the second time through this book was quite a different experience, even taking format into account; I knew ahead of time what some of the "big reveals" were going to be (what Jason's purple shirt signifies, for example) so I could appreciate Riordan's craftsmanship in his foreshadowing of those events more than I could the first time. Worth a first, and then a second read!