This short paper, "To What Intent Does God Afflict Us With Sickness?", is an excellent, brief resource that addresses common questions surrounding the reality of suffering and God's omnipotence. The PDF of the work is provided by 1580 Press, a site where one can find many other L-TOMs and other Lutheran works. I believe 1580 Press is run by Nathan Higgins.
Sihler was an important figure in the formation of the Missouri Synod and the establishment of the Fort Wayne Seminary. For a article on his life and work, see "Professor Wilhelm Sihler: Founding Father of Lutheranism in America and First President of Concordia Theological Seminary" by Lewis Spitz Jr.
In this brief paper, Sihler directly confronts the often troubling truth that God sends sickness to both unbelievers and believers. Straightforward assertion of the truth and childlike trust in the Word and promises of God are hallmarks of Missourian writing--especially from this era. And on this front, Sihler does not disappoint. When discussing God sending sicknesses to unbelievers, he writes: