I would strongly suggest R’ Samson Raphael Hirsh’s “The Nineteen Letters.” Is is a brilliant explanation, of the purpose of creation, the role of humanity on the world, the role of Jews in humanity, and the role of Torah and halacha in Jewish life.
Horeb, by the same author is also absolutely brilliant. In it, R’ hirsch explains the practical reasons for the mitzvos – why Hashem commands us to do X or not do Y, when He could have chosen to command and prohibit something else entirely. It is wonderful for understanding what our following halacha is supposed to do for us in terms of bettering our daily existence.
Horeb is a bit difficult at times, so I would say start with the Nineteen Letters, whioch really sets a basic hashkafic framework which he then places all the mitzvos into in Horeb.