“Valley Forge” by Bob Drury and Tom Clavin is a seasonally-appropriate account of the longest winter of the American Revolution. In 1777, after being beaten at Brandywine and Germantown, Washington’s army abandoned Philadelphia. Congress fled to Lancaster, and the army took up winter quarters at Valley Forge. Equal measures of privation, disease and dissent threatened the army’s survival. Drury tells how Washington kept the army together and prepared them to fight another day.