Lincoln and the Triumph of the Nation: Constitutional Conflict in the American Civil War by Mark Neely (UNC Press, 2011).
I am around 100 pages into this book and liking it. Neely examines the usual issues -- secession, habeas corpus, emancipation, conscription, etc. -- but a major part of what makes his book unusual is the in depth look at the northern pamphlets published during the war both in support and of in opposition to the policies of the Lincoln administration. In general, the pamphlets in question are those offering serious legal arguments, not merely partisan emotional pleas, and Neely's analysis (in the interrelated contexts of legal and historical precedent) of their merits is very much worth reading. The book also devotes a large section to a similar scrutinization of the Confederate constitution.