Riordan's series have been on bestseller lists since the first book's publication in 2005. The collective "Percy Jackson" series is currently at No. 3 on the New York Times bestseller list for Children's Series.
In addition to other works, the author has also written two other series that deal with mythology. One, the "Heroes of Olympus" books, is set in the same universe as the "Percy" series and follows Jason, who finds out he is the son of Zeus. Various characters from the "Percy" series, including the boy himself and Annabeth, appear in "Heroes." The "Heroes" series currently consists of three books with a fourth, "The House of Hades," scheduled for release this October.
Another Riordan series known as the "Kane Chronicles" centers on siblings Sadie and Carter, who discover they are descendants of the pharaohs and must do battle with Egyptian gods such as Set. The third and what is believed to be the last book was released in 2012.
In a short story titled "The Son of Sobek" which came out this past May, Riordan had his heroes Percy and Carter do battle.
The author has stated he'll also be writing a series centered around Norse gods but says not to look for it until possibly 2015.