It seems to be one of the many unique aspects of JRR Tolkien as a writer, that nearly all of the vast numbers of people who have read him, have read only The Hobbit and The Lord of the Rings. They have not read any of the smaller works published during his lifetime, nor have they read any of the couple of dozen books of extra material published by Christopher Tolkien after his death.
Indeed, I would guess that a very large majority of those who read, and loved, and frequently re-read the LotR; have never read the Appendices of that book (and certainly do not re-read them).
I don't find this particularly surprising (not least because I am describing my wife!) but it does demonstrate the gulf between the majority of Tolkien lovers (not 'fans') and the minority such as myself who - from the age of 13 - read all of his fiction ASAP, and at least 'tackle' almost every scrap of writing (and drawing) he produced (including sitting down and reading the entire, in parts almost day by day, chronology of his life published in the JRRT Companion and Guide by Scull and Hammond.
(I have probably not read all his writings about the invented languages, because I can't understand them - that is, I looked at the words... but that doesn't really count as 'reading'.)
I think this is very interesting and distinctive. There are some writers - indeed most famous or influential writers - who are known for one book. (To be a 'one hit wonder' is The Norm, especially among dramatists - and nothing to be sneezed at!) There are others who are famous for several or lots of books/ works - Shakespeare and Shaw, Dickens and Austen... This applies to minor writers as well as major - either just one genre classic, or a great mass of popular works.
But to be very famous, very beloved, and very influential on the basis of two books; is perhaps unique?
Note: The only other example I can think of is RB Sheridan with two classic plays: The Rivals and School for Scandal.