There's a couple of "Paletas and Agua Frescas" cookbooks on Amazon. (This (eG-friendly Amazon.com link) and This (eG-friendly Amazon.com link). There's an agua fresca recipe book in German and a kid's book in Spanish also.
The traditional flavors around here are Horchata (almond,rice,cinnamon), Jamaica (hibiscus), Tamarindo, Sandia (watermelon), Pina (pineapple) and my fave Pepino (cucumber lime). Those books have those and far beyond combos from my quick perusal.