"Nearly ten years has passed since the close of the civil war in America, and yet no satisfactory history thereof is accessible to the public; nor should any be attempted until the Government has published, and placed within the reach of students, the abundant materials that are buried in the War Department at Washington. These are in the process of compilation; but, at the rate of progress for the last ten years, it is probable that a new century will come before they are published and circulated, with full indexes to enable the historian to make a judicious selection of materials." General William T. Sherman St. Louis, Missouri, January 21, 1875

The most complete text composed of primary documentation I've found regarding the use of Siege or Seacoast Mortars during the period in question is contained in the "Official Records of the Union and Confederate Navies in the War of the Rebellion". Congress approved an act on June 27, 1874 appropriating funds for the amassing of "all reports, letters , telegrams, and general orders not heretofore copied or printed, and properly arranged in chronological order." This work in progress went into print on approval in 1880 and final text was printed in 1920. The running serial of the volumes is seventy six and is ordered into four series. In order to search an event by the time it occured one might want to print out the "Table Showing Volumes Pertaining to Contemperaneous Operations." The complete document is internet accessable if one doesn't wish to spend a pile of resource on original volumes. Exceedingly collectable. I include the internet links to the text. See Volume 18 for the operations on the Mississippi, the largest of the Seige Operations and pulled directly from the RN. The Siege of Ft. Pulaski on the Savannah River was the first and reading Sam Francis DuPont's letter to his wife (Not contained in the ORN.) tells a tale of despair. The initial shells were not cast concentric and fuze times were erratic. Materials were corrected in time for operations on the river, with devestating effect. Regards Henry E. Whittle Bradenton, Florida 3/27/09

http://cdl.library.cornell.edu/moa/moa_browse.html

Volume 18: West Gulf Blockading Squadron (February 21, 1862 - July 14, 1862). 1904.
Author: United States. Naval War Records Office Title: Official Records of the Union and Confederate Navies in the War of the Rebellion Series: Office memoranda (United States. Naval War Records Office) Publisher: Govt. Print. Off. Place of Publication: Washington MoA Volumes: Series I, vols. 1-27; Series II, vols 1-3 (1894 - 1922)