If you're interested in more information about the Percy Jackson series, drop by Rick Riordan's site at:
What's cool about this site is that it shows the cover art, talks about each book in the series, and offers links to video of Rick Riordan reading from the series.
I had mistakenly thought that Book 4 might be the last of the Percy Jackson books, having read in Horn Book that it was the "penultimate" book of the series. I need to use the dictionary more often. Turns out "penultimate" means "next to last". Good news for all of us Percy Jackson fans!