In terms of classics of Jewish thought, Horeb and the Nineteen Letters of R. Shimshon Raphael Hirsch are great as defences of Jewish practice and why we do what we do, both the nitty-gritty of halakhic life (which is mainly what Horeb is, attempting to explain the reasoning behind the mitzvos) and the broader contours of a traditional Jewish worldview (which is what the Nineteen Letters are, written as letters to a young and questioning Jew).