This quest can be acquired from Brother Nicodemus at the Sasau monastery. He is looking to establish a new Herbarium, and requires you to find one specimen of each of the following plants:
-- Nettle -- Dandelion -- Comfrey -- Sage -- Herb Paris -- Poppy -- Mint -- Chamomile -- St. Jon's Wort -- Eyebright -- Marigold -- Thistle -- Belladonna -- Wormwood -- Valerian
Each of the plants can either be picked in the wild or be purchased from Nicodemus himself, herbalists, traders and apothecaries.
In addition, Nicodemus requires an old book of any sort, which can be purchased from a trader if you do not already have one in your inventory.
Once you have all of the items, return to Brother Nicodemus to complete the quest.