Stehle's Door by William M. O'Brien Jr.
Approx. 130,440 words.
Published on July 11, 2011 by
Paul and Alex Zunker find out soon enough that their Civil War trophy, a packet of letters, is a curse. It seems they are a “trigger” to a dimensional doorway that ushers in a procession of hideous supernatural manifestations that threatens the Zunker family, particularly Alex.