Destiny's Landfall: A History of Guam by Robert F. Rogers is the most thorough single-volume history of the island, covering everything from the first Chamorro settlements through Spanish colonialism, the war years, and postwar American rule. Dry in places but genuinely informative — essential reading if you want to understand why Guam is the way it is.
The Island of the Colorblind by Oliver Sacks visits Pingelap in Micronesia rather than Guam specifically, but it's the best English-language book about the broader region — curious, humane, and beautifully written. It gives you a feel for what the Pacific archipelagos are actually like.
Fiesta de Guam edited by Patrick Flores is a collection of short writing and oral history from Chamorro perspectives, harder to find but worth seeking out for a view from inside rather than outside.
For a practical regional guide, Moon Micronesia (various editions) covers Guam in depth alongside Saipan, Palau, and the wider region, and is useful for planning if you're island-hopping.