Fr. Miller is very knowledgeable about the RC chaplaincy and is working on a sequel of sorts to Both Prayed to the Same God, which will provide a roster, with brief bios, of the RC chaplains in Union and Confederate service. Anyone interested in that subject might also want to look for an edition of David P. Conyngham, Soldiers of the Cross..., ed. David Endres and William Kurtz (due out in April from University of Notre Dame Press); its emphasis is on Catholic priests and religious (mostly Irish) who served the Union, though he includes a few others. The journal of Fr. James Sheeran, the southern Redemptorist chaplain, has been newly edited by Patrick J. Hayes (Catholic University of American Press, 2016), with excellent notes; Fr. Hippolyte Gache's war letters are still available in the fine edition of Cornelius Buckley, titled Frenchman, Chaplain, Rebel (University of Alabama Press). My own book on Fr. Darius Hubert, the "soldier priest of the Confederacy," will hopefully be added to this shelf before too long.
There are quite a few book-length studies of the CW chaplaincy (north and south, RC and Protestant), of varying quality, and some individual bios and editions of letters and papers. Not to mention a few excellent theses, dissertations, and articles. But this has been, overall, an under-studied aspect of the war.