I've had my problems here and there with Russel Beatie's Army of the Potomac books, but it is still my favorite series of recent memory, truly epic in scope and insight. In my opinion, its pages contain the best overall portrayal and analysis of the early war in the eastern theater from the Union perspective.
Between 2002 and 2007, the author was able to churn out three volumes, the first two published by Da Capo and the third by Savas Beatie:
Vol. I - The Army of the Potomac: Birth of Command, November 1860-September 1861 (2002).
Vol. II - Army of the Potomac, Volume II: McClellan Takes Command, September 1861-February 1862 (2004).
Vol. III - Army of the Potomac: McClellan's First Campaign, March - May 1862 (2007).
It has now been 5 1/2 years since Volume III was published and still no hint of when the next one will appear. Given the extraordinary ambition of the series, especially one launched so late in active life, it comes as no surprise the original pace could not be continued. Even if we never see a IV (I am hoping Beatie will at least get through the Seven Days and 2nd Bull Run), the work done so far comprises a substantial career achievement.