"1901" by Robert Conroy is based upon the German war plan for the US at the turn of the 20th century, except in the novel the plan is actually carried out. It's an interesting premise and a good read. I enjoyed seeing many familiar figures out of their normal historic context - a young Chester Nimitz, James Longstreet returning from retirement to lead the Army, and Theodore Roosevelt as a wartime president to name a few.
The book has been out of print for several years but it's well worth hunting down a copy if one is interested in the era, subject matter, or speculative fiction in general.
Already done :-). found it for next to nothing in good condition on some internet site the name of which escapes me. Might have been bookfinder.com or something like that.
The worst part of the book is the arbitrary way the war starts; the Germans just randomly attack. They would have at least tried to engineer a crisis, or fake a casus belli.
The torpedo attack in NY Harbor, and the Apaches brought into the NYC suburbs running around terrorizing German troops by killing and mutilating them, was quite cool though.