According to the Shulchan Aruch Orach Chayim 554:1-3 one is permitted to study the book of Iyov/Job on Tisha b'Av. The book of Iyov does not seem to have any connection to the destruction of the Temple which is being mourned on Tisha b'Av nor does it have any connection to any other sad event in Jewish history. The majority of the book are a series of arguments between Iyov and his friends regarding how God administers punishment in the world.
I would like to understand why learning this book is permitted on Tisha b'Av.