I've been searching for this long, long out-of-print title from the great Edwin C. Bearss for years. It is a rare find and a bit too expensive on the secondary market, but I finally obtained one through ILL (strangely enough, it is a signed copy from a monastery library about an hour's drive from me!).
Decision in Mississippi (1962: Mississippi Commission on the War Between the States) is typical Ed in terms of style, depth, and content. This book precedes his monumental 3 vol. history of the Vicksburg Campaign and about three-fourths of DIM's content is made up of chapters that would later appear in the trilogy. What you won't find anywhere else is a 60-page article on the battle of Iuka, and chapters dealing with various Federal cavalry raids in SW Mississippi during the late summer and fall of 1864. The Battle of Woodville is detailed here along with the failed Union mounted raids on the Mobile and Ohio depots during Hood's Tennessee campaign.
Another thing of note is how serious Civil War military history was treated by local and state organizations in this period long before I was born, when these commissions would fund the research and writing of a 630 page scholarly CW military history. As an avid reader and collector of Bearss's work it is clear how much of it has benefitted from these groups, undoubtedly spawned by the Centennial.