Question: What is the name of the first book in the series that precedes The Heroes of Olympus?

Related documents: [0]  [DOC] Percy Jackson & the Olympians, often shortened to Percy Jackson, is a pentalogy of adventure and mythological fiction books written by American author Rick Riordan, and the first book series in the Camp Half-Blood Chronicles. Five supplementary books, along with three graphic novels, have also been released. More than 45 million copies of the books have been sold in more than 35 countries. [END] 
[1]  [DOC] The Heroes of Olympus is a pentalogy of adventure, mythological fiction and fantasy novels by American author Rick Riordan that collectively form a sequel series to the "Percy Jackson & the Olympians" series. This series details a conflict between Greek and Roman demigods and Gaea, the earth goddess. Riordan introduces Roman mythology into his sequel series as well as several new characters. The first book of the series, "The Lost Hero", was published on October 12, 2010. The final entry in the series, "The Blood of Olympus", was published on October 7, 2014. [END]  
Answer: Camp Half-Blood Chronicles
Camp Half-Blood Chronicles