More Guns, Less Crime.
In my opinion THE two volumes that form the core of a library of 2A volumes are, first, The Founders' View of the Right to Bear Arms by David E. Young, and, second, his The Origin of the Second Amendment: A Documentary History of the Bill of Rights 1787-1792. The second volume is a compend of texts from primary documents cited in the first volume. These volumes aren't cheap, but they're worth it. Young was cited by 23 parties filing amicus briefs in Heller--on both sides of the issue! Young's discussion is deep and broad. The historical texts quoted are invaluable. These volumes are like buying high end optics--"buy the best, cry once."
A valuable secondary volume is Stephen P. Halbrook's The Founders' Second Amendment: Origins of the Right to Bear Arms.