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

Context articles: The Lost Hero
The House of Hades
The Blood of Olympus
Percy Jackson
The Trials of Apollo
Percy Jackson &amp; the Olympians
Tarzan the Magnificent (novel)
Demon Thief
The Heroes of Olympus
Path to Truth

The Lost Hero is a 2010 fantasy-adventure novel written by Rick Riordan and is based on Greek and Roman mythology.
 It is the first book in the series "The Heroes of Olympus", the second series about demigods and Camp Half-Blood.
 It was preceded by the "Percy Jackson & the Olympians" series, which focused solely on Greek mythology.
The House of Hades is a 2013 fantasy novel based on Greek and Roman mythology written by Rick Riordan.
 It is the fourth book in "The Heroes of Olympus" series, which succeeds the "Percy Jackson & the Olympians" series.
 It is narrated in third person by the seven demigods of the "Prophecy of Seven".
 The demigods are continuing on their quest from the end of "The Mark of Athena" to close the Doors of Death and slow the progress of the goddess Gaea's army in the mortal world.
The Blood of Olympus is a young adult novel written by American author Rick Riordan and is the fifth and final book in "The Heroes of Olympus" series.
 It was released on October 7, 2014.
 The seven demigods of the Prophecy of Seven—Percy Jackson, Annabeth Chase, Jason Grace, Leo Valdez, Piper McLean, Hazel Levesque, and Frank Zhang—go on their final adventure to defeat Gaea while Nico di Angelo, Reyna Avila Ramírez-Arellano, and Coach Gleeson Hedge attempt to bring the Athena Parthenos to Camp Half-Blood in order to prevent a war between the Roman and Greek demigods.
 The chapters are written from the perspectives of Jason, Piper, Leo, Reyna, and Nico, making it the first time in the series that someone other than one of the seven demigods of the prophecy is the viewpoint character.
 It is the second time that Percy appears without being a viewpoint character (first being in "The Lost Hero").
Perseus "Percy" Jackson is a fictional character, the title character and narrator of Rick Riordan's "Percy Jackson & the Olympians" series.
 He is also one of seven main protagonists of the sequel series "The Heroes of Olympus", appearing in every book except "The Lost Hero," and appears in the ongoing "Trials of Apollo" series, making him one of the few characters to appear in all three series of the Camp Half-Blood chronicles.
 He has also been a narrator and protagonist in Riordan's Greco-Roman/Egyptian crossover stories, part of the "Demigods and Magicians" collection.
 The character serves as the narrator in "Percy Jackson's Greek Gods" and "Percy Jackson's Greek Heroes", also by Rick Riordan.
The Trials of Apollo is a series of fantasy adventure mythological fiction novels written by American author Rick Riordan that collectively form a sequel to the "Heroes of Olympus" series.
 The first book in the series, "The Hidden Oracle", was released on May 3, 2016.
 The second book in the series, "The Dark Prophecy", was released on May 2, 2017.
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.
Tarzan the Magnificent is a book by American writer Edgar Rice Burroughs, the twenty-first in his series of books about the title character Tarzan.
 It was originally published as two separate stories serialized in different pulp magazines; "Tarzan and the Magic Men" in "Argosy" from September to October, 1936, and "Tarzan and the Elephant Men" in "Blue Book" from November 1937 to January 1938.
 The two stories were combined under the title "Tarzan the Magnificent" in the first book edition, published in 1939 by Edgar Rice Burroughs, Inc. In order of writing, the book follows "Tarzan's Quest" and precedes "Tarzan and the Forbidden City".
 In order of book publication it falls between the latter and "Tarzan and the Foreign Legion".
 The novel's plot bears no relation to that of the 1960 film of the same title.
Demon Thief is a book in Darren Shan's "Demonata" series.
 Though it is the second book in the series, it is a prequel to "Lord Loss", the first book in the series.
 The protagonist is also different from that of the first book.
 The narrator here is a new character called Kernel Fleck, as opposed to Grubbs Grady, the protagonist of the first book.
 Demon Thief takes place about thirty years before "Lord Loss" so most of the characters from the first book did not appear, though a few did.
 It was also actually the sixth book of the Demonata to be written, although it was the second released.
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.
Path to Truth is the first book of the Jedi Quest series, set in the "Star Wars" universe.
 It is written by Jude Watson and takes place between the events of "" and "".
 It precedes "The Way of the Apprentice", which is the official first book in the series, as it is the first to be sequentially numbered.