The Titan's Curse is written by Rick RiordanRick Riordan wrote Vespers Rising
[Human]: Could you recommend to me a book similar to The Titan's Curse? [Assistant]: Sure, do you like the author, Rick Riordan and the genre Young-adult fiction?  [Human]: I love Riordan and his classic book Vespers Rising! 
If you enjoyed Vespers Rising, I recommend checking out J.K. Rowling's Harry Potter series. Both books are set in magical worlds and have equally compelling storylines that will keep you hooked.