Portallas is a YA series of fantasy adventure novels written by New York Times & USA Today best seller Christopher D. Morgan.
Each book in the Portallas series plays out in a different world/environment. Forestium is a heavily forested environment, Archipelago is a group of desert islands, etc. The series chronicles the adventures of Joshua, the main protagonist, along with three of his friends, as they battle with the main antagonist.
Along with the main novels in the series, Christopher D. Morgan is writing a short story/novella for each main character. Each of these goes into the back-story of the featured character and provides some explanation for how they came to be in the situation we find them when we meet them for the first time in Joshua and the Magical Forest.
The Portallas universe, the characters, magical creatures, etc., are described in great detail on the series website: http://portallas.com/