I came across these fun websites for authors Riodan, Delaney, and Flanagan.
Riodan's site offers a section on Percy's Adventures as well as a section for More Worlds to Explore.
Delaney's website is suitably named The Spook's Mistake. The site offers visitors a chance to see if they have what it takes to be a Spook's Apprentice. There is also a place to sign up to receive the Spook's email newsletter.
Flanagan's website is The Ranger's Apprentice. The site offers in-depth accounts of the characters, heroes and villains alike, as well as maps of the different countries traveled through in the series.
Adults and young readers will enjoy exploring each of these sites.