William T. Vollmann is without question the most ambitious, indulgent American writer of his generation.

Fortunately, he has all the talent, singularity of voice and dedication required to live up to that ambition. Vollman has written 12 books of fiction, including the National Book Award-winning "Europe Central."

"Imperial" is his seventh nonfiction work, adding to an output that includes "Rising Up and Rising Down," a seven-volume, 3,299-page treatise on the history of violence.

The thing is, despite the prodigious output, Vollman has written few bad books. "Imperial" is an exhaustively researched macro history of California's Imperial Valley, a desert region that became an agricultural powerhouse after canals brought irrigation water in 1901.

It's now a major source of fruits, vegetables, cotton, grain and, because it abuts Mexico, immigration tensions.

Vollmann doesn't limit his survey to that physical location. His subjects are the borders -- physical, psychic, economic, even sexual -- that separate the United States from Mexico. "Imperial" provides an amazing and unparalleled contribution to our understanding of who and what we are as a nation.

For Vollman, the Imperial Valley is a microcosm for all of North America:

"When I began to study the history of the period, my mind remained unbiased by knowledge. All I knew was that somehow Imperial County had altered from being one of the richest bits of farmland in the United States to the poorest county in California, and I couldn't fathom how."

What we learn is not always pretty. That's a large part of what makes "Imperial" so impressive. It's an astounding book that raises the level of the rhetorical tools available to historians -- and, therefore, our expectations of them.

Vollmann is no armchair reporter, but a throwback to the days of the author-adventurer putting his own neck in jeopardy to get at vital truths of our society. The sections of the book that are lived rather than reported are particularly profound.

The book's biggest payoff comes in the chapter titled "The Maquiladoras." Vollmann describes his clandestine efforts to film working conditions inside American-owned factories in Mexico known for their appalling mistreatment of employees.

It turns out that many of the workers are grateful for the jobs those factories provide. Here, Vollmann is at his best because he's willing to lay aside values and prejudices out of respect for other points of view. He comes to appreciate that there's no consensus among the workers about their own conditions.

That approach characterizes Vollmann's balanced research, and the organization of all this material into a cohesive, compelling narrative is a marvel in itself.

The obsessive detail of "Imperial" will test the patience of Vollmann's most ardent admirers. You can trust me on that because I'm one of them.

There's a ton to admire about this book (and maybe a ton for some readers to skim past), and it's required reading for anyone interested in notions of identity played out every day on the U.S.-Mexican border.

In the many, many hours it took me to read and review it, I came to believe that this book is ultimately a meticulously constructed time capsule. When future generations look back at our era to figure out what went wrong, "Imperial" will be waiting.