One of the most influential philosophers of the 19th century (although not during his lifetime) was Friedrich Nietzsche. I have read a number of his books, and while he has an interesting style, I struggled to get his full meaning.
What I have found helpful is reading Nietzsche: Philosopher, Antichrist by Walter Kaufmann. I felt like I finally had a sense of what Nietzsche was trying to say.
One of the problems with trying to understand Nietzsche was that near the end of his life, his sister took control over his publishing. She had her own agenda and was not gifted enough to understand what her brother was doing. Very often we read Nietzsche through the lens of his sister and this leads to all sorts of misunderstandings.
Kaufmann provides a helpful summary of Nietzsche’s life and thought. He breaks down the major themes and demonstrates what Nietzsche really believed. As a Christian, I especially appreciated the chapter on Christianity and Nietzsche’s complex attitude toward Jesus.
If you are interested in Nietzsche at all, I definitely recommend Nietzsche by Walter Kaufmann as your starting point.