"It's a collection of letters and diaries about the Civil War in Washington County," said Keller, an historian and broadcaster for WHAG radio.
"Crossroads" complements and reinforces his first two books, "Roster of Civil War Soldiers from Washington County" and "Events of the Civil War in Washington County, Maryland," he said.
The richly detailed and often poignant material comes from historical societies, family archives and research at places like the Army War College in Carlisle, Pa., Keller said.
Among the book's offerings are letters from Private Walter Scott Myers to his mother back home in Clear Spring and Angela Davis' letters to her nieces recounting "the tramp, tramp, tramp of the soldiers at night" outside her Funkstown home and the raids on her general store.
At the end of the book there is a list of sick and wounded soldiers who were hospitalized in Hagerstown and Williamsport and some local legends about the war.
While most people know about the "bloodbath" at the Battle of Antietam, Washington County's role during the rest of the Civil War often is overlooked, Keller said.
In early July, 1863, there was fighting in Smithsburg, Hagerstown, Williamsport, Boonsboro and Funkstown as the Confederate Army retreated from the Battle of Gettysburg, Keller said.
Lee's troops were backed up in Washington County waiting for the Potomac River at Williamsport to recede enough for them to cross it, he said.
The Civil War stirred tensions in the county. About 75 percent of Hagerstown was sympathetic to the Union but "there were still some strong feelings for the South," Keller said.
While Hancock was mostly pro-Confederate, Clear Spring was "markedly" pro-Union, he said.
Keller, 65, teaches classes on the Civil War for Elderhostel at Hagerstown Junior College and the Institute for Learning in Retirement, he said.
"Crossroads of War" is due out in September.
Keller's first book, the roster of local Civil War soldiers, will be reprinted this fall, he said.
Keller said he has two more books in the works - one on Confederate Gen. Thomas Rosser, commander of the successful Laurel Brigade, and the other on Isaac Shelby, the first governor of the state of Kentucky, who grew up in Clear Spring and fought in the American Revolution and the War of 1812.