Rick Riordan's The Lightning Thief was originally written as a bedtime story for his son who suffered from ADHD and dyslexia.
When Riordan's eldest son Haley was in second grade, he asked his father to read him stories of Greek mythology. Once Rick ran out of the real myths, he decided to create his own! Haley enjoyed these modern retellings so much that he urged his father to publish them in a book! The fruit of these labors is the Percy Jackson and the Olympians series (think Harry Potter, but with Greek mythology instead of wizardry). The main character of the series, Percy, was written to have ADHD and dyslexia so Haley could have someone to whome he could relate!