So far in my journeys I've come across a bunch of Toorum's notes. Are they purely flavor text or do they serve some purpose?
They are purely flavor, they do nothing more. My previous answer talked about an achievement that was incorrect, this is not true. The achievement I thought I unlocked by finding all the notes was: (spoiler)
For finding Toorum's body on level 10, it just so happens that near his body contains his last 2 notes.
which unlocks the "Buddies with Toorum" achievement. This allows you to play as Toorum in future playthroughs. More information on that can be found here.