There may be a few different translations or editions out. My copy is from Franciscan University Press, Steubenville, Ohio, 1996. I had to buy it, because it seemed like every good apologist or convert I was reading was deeply influenced by it.
It was written in German back in the 1930’s. I found there many of the sources for good ideas that have been repeated by many others since then.
If only they would pass out this book in the RCIA program, instead of the nonsense that is often found in our diocese.