Harvard's Civil War is a regimental history of the 20th Massachusetts Infantry Regiment. It was written by Richard F. Miller and can be bought at Amazon.com, Barnes and Noble or abebooks.com. There is also a nice summery of the book at this link:
http://www.upne.com/1-58465-505-4.html
There is also another 20th Massachusetts regimental history written by George A. Bruce called,
The Twentieth Regiment of Massachusetts Volunteer Infantry 1861 - 1865. This book is a little harder to find but, I believe you can buy it at abebooks.com.
Both are excellent books.
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Harvard's Civil War 
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