George Washington Volume IV by Douglas Southall Freeman

This is probably the largest volume in the series, covering the end of the siege of Boston (early 1776) through the disastrous evacuations of New York City and much of New Jersey, and concluding with the abandonment of Philadelphia (1777).  The breadth of detail covered in this time period is vast, so I won't attempt to retell the story except in broad strokes.

In quick synopsis, Washington earned roaring triumph by forcing the British to abandon their occupation of Boston, quickly followed by a low point with the lightning-fast loss of New York City.  The reversal would have crushed almost anyone, but not Washington, neither in public reputation or in his own sense of mission.

Washington's high point in the volume had to be the Americans' first major offensive victory, the dawn attack at Trenton, quickly followed by a similar success at Princeton.  These victories provided a much needed supercharge to the Revolution, its citizens, supporters, soldiers and its general.

The volume ends with the loss of Philadelphia and its capture by the British.  Throughout the volume, the potential loss of Philadelphia is frequently stated by Washington as synonymous with the loss of the Revolution itself.  When the event actually occurred, however, nothing could be further from the truth.

Although not happy to lose the city, the seat of the American Congress and symbol of the Revolution, it soon occurs to Washington that the cause of the Revolution is much broader than any one city.  In the literal sense, the Americans had removed all stores of value and Congress had long since evacuated the city.  The British conquered the ghost of Philadelphia, with nothing of strategic importance.

But the loss of Philly coupled with the survival of the struggled proved to Washington that winning the war was not about territory: It was about outlasting British resolve to hold it.  He didn't need to win back every piece of American land; he simply needed to fight until the British were done fighting.  No one city, no group of cities or pieces of territory could signify a victory.

I believe this was a turning point in the war and in Washington's approach to it.  We shall see in Volume V.

Lastly, two points:  

It's been interesting to see how much of the story is composed of the struggle for supplies.  America had so little manufacturing power at the time, it's almost impossible to believe.  Many men spent the winter at Valley Forge without ANY clothing at all.  How was that possible?  Organization, planning, infrastructure, transport, chain of supply.  I've read repeatedly throughout history that that's how wars are won, and in fact, lost.  How the Americans overcame this deficiency is amazing.

A second subject of many, many pages is the squabbling among American (and foreigners in its service) officers, upset over perceived inequalities in rank, pay and prestige.  Washington apparently spent shockingly large amounts of his time dealing with these petty issues.  Again, it's amazing that he was able to overcome such insubordination and lack of patriotism.

Update:

Now several weeks after publishing this post I've run across a quote that supports my note above about Washington's new perspective on how to win the War.  From Chapter Four of Founding Brothers by Joseph J. Ellis:

"Although he actually lost more battles than he  (Washington) won, and although he spent the first two years of the war making costly tactical mistakes that nearly lost the American Revolution at its very start, by 1778 he had reached an elemental understanding of his military strategy; namely, that captured ground - what he termed "a war of posts" - was virtually meaningless.  The strategic key was the Continental Army.  If it remained intact as an effective fighting force, the American Revolution remained alive.  The British army could occupy Boston, New York, and Philadelphia, and it did.  The British navy could blockade and bombard American seaports with impunity, and it did.  The Continental Congress could be driven from one location to another like a covey of pigeons, and it was.  But as long as Washington held the Continental Army together, the British could not win the war, which in turn meant that they would eventually lose it."