Krakatoa: The Day the World Exploded: August 27, 1883 is by Simon Winchester. A fascinating look at the world's worst volcanic explosion in modern times. Winchester not only walks you through the science of volcanic eruptions in a readable and understandable way; he also looks at the human and ecological toll of the eruption. A well written and absorbing read.
Ratings: 9th grade - 9 out of 10 - AC (some mature thematic content).