This book was far from a perfect book -- there were typographical and editorial errors, some quite blatant and relating to verb tenses, which muddied the text -- but the book was the perfect ending to a very good series.
Rick Riordan seemed to struggle a bit in the first book because the protagonist felt, at time, far older than his stated twelve years. Riordan, is very good at writing characters that are in their mid-teens. He is very good at giving motivation. He is very good at showing shades of grey: His "heroes" are not without flaws and vices, and his "villains" are not without virtues or necessarily beyond redemption.
This was an excellent ending to the series, and did make me want to start reading the next.