Keith Kelly 
The Magic Blanket Fort [EPUB ebook] 

When Ben and his Grandson Charlie build a fort out of blankets one evening, little Charlie feels the fort is magic. They fall asleep in the fort, and little do they know that the next morning they will wake up in a magical land.
Every two hundred years, humans are brought to this land for a very specific purpose. After one of their new friends goes missing, the two begin a journey of discovery and purpose.
On this adventure of a lifetime through a land of kings, kingdoms and excitement, Ben and his grandson meet many inhabitants of this strange realm, and make friends that will last a lifetime.