Matthew K. Manning 
Revenge of the One-Trick Pony [PDF ebook] 

The secret of the Rainbow-Barfing Unicorns is out and that makes Xander Stone sick! Twelve-year-old Xander is the only person on Earth who knows about the Rainbow-Barfing Unicorns. Except, of course, for a rich and crazy billionaire who will stop at nothing to capture these zombielike, upchucking creatures and get even richer and crazier! Him too. The dashing, naively heroic Xander is the only hope for the Rainbow-Barfing Unicorns. (So yeah, the odds aren’t great.)