A little sleuthing may suggest a reason why books 2 and 3 did not reach publication.
3000 copies of Book 1 were printed in 1977. It was reasonably handsome, but not lavish. It had 221 pages. The price was $35, equivalent (according to this cool inflation adjustment website
) to $129.87 in 2010.
Book One, expected to be "by far the most commercial," simply may not have been a very brisk seller.
In the March 1995 Genii (vol 58, p 452) -- 18 years later -- we encounter this 1/3 page monument-style announcement: