A Tolkienist is someone who studies the work of J.R.R. Tolkien: this usually refers to students of the Elvish languages. A Tolkienist can also be described as a hard-core fan of Tolkien's work, one who studies the work with the same amount of interest (or more) that others study non-fictional subjects. Many fans prefer this term, as it isn't limited to The Lord of the Rings.
See also Tolkien Fandom