I had to farm 170 Unknown Intestines to get 10 Fine ones (and 160 of the blue ones).

Drop rate for Unknown Intestines seems to be about 10-15%, so get ready to spend 1-2 hours farming those.

I recommend farming this zone:

Had to farm 210 Unknown Intestines to get the 10 fine ones.

I farmed 70 intestines before I received 1 fine intestine. Prepare for a long grind to receive this Tome achievement.