When The Silmarillion was published in 1977, I had been utterly immersed in Tolkien's work for five years with an intensity that only teenaged fans can muster. To say I was 'looking forward to' its publication is a gross understatement - I had even exchanged letters with Tolkien just before he died asking when the book was coming.
Then it arrived in the bookshops, and I bought it (in hardback - expensive when relying on pocket money) immediately... and yet I found (to my own astonishment, and indeed embarrassment) that despite expectations - I enjoyed it so little that I could not even finish it.
I have since read it through, and also listened twice to the whole thing on audiobook; but I still find The Silmarillion Tolkien's worst book - and the only one I don't spontaneously love; deeply flawed in many ways - although with some excellent sections (such as the account of Numenor - which is wonderful in its way).
But then the 1977 Silmarillion is not really JRR Tolkien's work, but a compilation and edited mosaic of his unpublished texts made by Christopher Tolkien and Guy Gavriel Kay; who were primarily trying to remove inconsistencies (internal and between the Silmarillion text and the Lord of the Rings) - and literary quality was the main casualty. Christopher has since expressed regret at having published the 1977 Silmarillion in the way he did - and has since produced the wonderful 12 volume History of Middle Earth (with all of the Silmarillion texts and more - but in better versions) preceded by the marvellous Unfinished Tales.
My other great disappointment was Robert M Pirsig's Lila: an enquiry into morals - which was also a long-awaited follow-up; to Zen and the Art of Motorcycle Maintenance: an enquiry into values (1974). I had really loved ZAMM, re-read it multiple times, and had also exchanged letters with its author - in relation to my publishing one of the earlier scholarly essays on the subject.
ZAMM did not change my life as much or as deeply as Lord of the Rings, because no book ever has - but it did change my life, and was certainly one of the main books of my early adult life (perhaps a dubious distinction?) - and it is a book I still find very enjoyable and valuable; although now I find its 'message' (being ultimately atheistic, un-Christian) to be actually rather dangerous in the sense of getting what feels like quite close to a satisfactory philosophy of life but not getting to the necessary destination. This means it is easy to 'get stuck' on the book; holding an incomplete, unsatisfying and ultimately nihilistic world view. At any rate, this happened to me.
But no other philosophical text had made anything like the sustained impact of ZAMM; and in 1991 my life was extremely unsatisfactory and I was really seeking some guidance - so as soon as I saw Lila and bought it; and as soon as I bought it I cleared space in my life to read it straight through.
It is not a good book. While ZAMM is close to novelistic perfection in terms of its writing, structure, pacing, and the intrinsic interest of its parts; Lila is not - it is clunky, preachy, contrived - and has a general atmosphere of seediness and malaise which I find very unappealing (especially in comparison to the freshness and 'innocence' of ZAMM).
The book gives every indication of being squeezed-out of an unwilling and uninspired Pirsig with the 'help' of an editor - it is asif written by a different author than ZAMM.
I don't like the philosophical aspects of Lila any better. While accepting that the implicit premise that the William James-esque Pragmatism of ZAMM is incomplete and unsatisfactory, the proposed Metaphysics of Quality is equally ungrounded and untheistic; but dull and pedantic.
I picked Lila up a couple of days ago to check my reaction - and it has not changed: I have no inclination to re-read. What I loved about ZAMM is almost entirely absent from Lila.
With both The Silmarillion of 1977 and Lila of 1991; it was not merely a case of the books failing to live up to the supreme standards of their illustrious predecessors (a matter of 'regression to the mean', if you like); but of being books of a different kind altogether, and books lacking (almost entirely) exactly what it was that I most valued in the earlier work by the same author.
From my perspective, I would prefer that these two books had never been published.