The Lost Hero, the first novel in bestselling author Rick Riordan’s highly anticipated new series for middle-grade readers, Heroes of Olympus, was released to great fanfare on Tuesday, as Bookpeople in Austin, Tex. hosted what Riordan described last week as a “worldwide party.” The book launch, which began at 4 p.m. with food, chariot races, and rock climbing, drew an estimated 800 participants, some of whom were dressed in costume, with many more wearing orange Camp Half-Blood t-shirts.

The event was beamed to 10,000 more fans around the world by Disney via a one-hour live webcast, with Riordan reading from The Lost Hero before taking questions, both from his live audience as well as online.

Riordan concluded the simulcast by announcing that seven “lucky demigods” will be selected in a sweepstakes to attend an exclusive one-week session at Camp Half-Blood at Bookpeople in July 2011. Bookpeople has hosted several Camp Half-Bloods in recent summers; complete information on the sweepstakes can be found here.

The Lost Hero, a sequel to the Percy Jackson and the Olympians series, was released Tuesday with a 2.5 million-copy first print run. The second book of an anticipated five is scheduled for release in October 2011. Although Riordan would not reveal during Tuesday’s launch party the title of the second book in the Heroes of Olympus series, because “it’d be a big spoiler,” he did disclose, “By the time you finish this book, you’ll know where I’m going with book two.”

Riordan told a group of media representatives during a conference call last week that he decided to write a second series about young demi-gods at Camp Half-Blood because he had so many more Greek and Roman myths he hadn’t been able to explore and twist into tales for modern-day readers in the first series. He also conceded that he wanted to “give Percy Jackson fans what they still want.”

One book will be released each spring in his three-volume Kane Chronicles series for middle-grade readers, based on Egyptian mythology, and one book will be released each fall in the five-volume Heroes of Olympus series, which will mix Greek and Roman mythology.

The Heroes of Olympus series, which is set soon after the final volume in the Percy Jackson series, The Last Olympian, features three teenage demi-god main characters whose adventures begin when they go to Camp Half-Blood: Jason, Piper, and Leo.

While trying not to reveal too much about the story arc of the new series, Riordan did disclose that Percy Jackson, Annabel, Grover, and other favorites from the first series will appear as secondary characters in the Heroes of Olympus series.

“The world of the demi-gods is going to get turned on its head,” Riordan assured his conference call listeners. “New revelations will have an impact on the Half-Bloods.”

Riordan will be traveling to 12 U.S. cities on a two-week tour, before traveling to the U.K. for a one-week/five-city tour there.