The History of Middle Earth (HoME) is a series of 12 books (preceded by a similar book named Unfinished Tales) consisting of the writings of JRR Tolkien edited by his son Christopher.
I have been reading them for several years, slowly, off and on, in a piecemeal fashion - and with tremendous satisfaction.
I read Lord of the Rings (LotR) when I was thirteen and for the remainder of my school years re-read it several times, plus everything else by and about Tolkien which I could find. While I was at college I did not read Tolkien very often, but returned to him after finding a copy of Unfinished Tales in a rented cottage, buying a copy of the selected letters secondhand, and borrowing TA Shippey's Road to Middle Earth from the library.
The problem with LotR was that by age 18 I had learned it almost off-by-heart, and it had begun to lose its effect. But now I have seen the movies several times, listened to the radio dramatization, and - most important - been reading early drafts of LotR in the HoME plus much other material on the earlier mythologies - so I have thoroughly confused my memory between what is in the real and finished LotR and all these alternative versions and extra ideas.
I can therefore become completely 'lost' in Tolkien's world, on a long term basis!
Reading the letters, and Shippey, also opened my eyes to the profound intelligence, creativity and wisdom of Tolkien as a man - and his inspired prophesy - so there is a secondary 'philosophical' level which I now explore Tolkien's world (and there are several good secondary sources to help - perhaps the best being Verlyn Flieger's books).
Essentially, the HoME has become a way to overcome the habituation – the getting-used-to – which is a consequence of mere repetition. Because, when I return to re-read the ‘real’, finished, published Lord of the Rings; I find myself astonished by its quality and coherence – an experience which recaptures some of the freshness of a first reading.