While not written by the main creator, it's still pretty solid.

First half is about Giyu and his mission to the mountains. While the story is predictable there's a lot of great moments for Giyu's growth as a character plus some vicious kills. This is a moodier story with a lot of heavy themes. The second half is Rengoku, much more uplifting and of course we get to see Rengoku before he becomes a Hirasha so that's awesome. Both pretty fun side stories worth reading if you're a fan of the series.