Healer of the Water Monster by Brian Young (and sequel). Chapter Book.
On the surface, 11-year-old Nathan is like many other boys. His parents are divorced, and he’s a little upset with his father. His paternal grandmother, Nali, is supportive, and he’s eager to spend the summer with her on the Navajo reservation in New Mexico, working on his science fair project…Wandering into the desert, he encounters Pond, an ailing water monster. Adults cannot see Holy Beings from the creation stories, but as a child, Nathan can; with the help of a communication stone, he enters a world of Navajo cosmology. (glossary, author’s note) (Fiction. 10-14)” Source: Kirkus Reviews