Milli's Lexicon is a rare Off-hand Quest Reward.
This item refers to Millicent Serene, a person who is mentioned in the flavor text that unlocks the quest, the creator of the original uncorrupted Felvine, the Fruit of Fertility. Rabine Saturna mentions her several times, especially as such:
...It will be destroyed - if not for our continued safety from the demonic taint, then for the memory of Millicent Serene... Milli...
Patch 1.3.0 (07-Mar-2005): Added.