Filliman Tarlock's journal is obtained by searching the tree in the middle of Filliman's grotto south of Mort Myre Swamp during the Nature Spirit quest. The player must read the journal to find vague instructions on how Filliman can transcend his current ghostly form to become a nature spirit. The journal is not available from the tree after the quest. It is not found in the bookcase in a player-owned house.
Most of the writing is pretty uninteresting, but something inside refers to a nature spirit. The requirements for which are "something from nature", "something with faith", and "something of the spirit-to-become freely given". It's all pretty vague.