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 ...

I came across this teaching a while ago - it came from a reputable source. I am quite sure the source was pre-acharonim. Does anyone know who said it and where?