The book Kalacakra Tantra (yes, spelled here without the "h") evolved out of a series of talks that Geshe Ngawang Dhargyey presented at the Sakya Tegchen Choling center in Seattle over a couple of months in early 1982, following the first Kalachakra Initiation (Madison, WI in July '81). H.H. the Dalai Lama had encouraged the Geshe to travel to the U.S. to present background materials for students of the Vajrayana, and this was the result.
While I'm not sure what is so "secret" about the content of this, as compared to other books on the Kalachakra, it certainly was an eye-opener as far as plain talk about various things which all too often are presented in "symbolic" terms. It also is very up front about specific things to watch for in one's practice, and how to avoid various pitfalls. This was, of course, a fascinating glimpse into what it must be like to be working on these practices in a monastic setting.
Again, this presentation came from the very early days of the Kalachakra's spread to the west (prior to 1981 it had never been given outside of India or Tibet), and the Geshe keeps touching back on the points of how fortunate we are as Westerners to be able to have fairly easy access to high Lamas (such as my having been able to have taken five initiations, despite having virtually no background in "practice", from His Holiness the Dalai Lama), where in its native land, most lay practitioners would have had extreme difficulties in attaining this sort of contact, if they could manage it at all.
While this is listed on the Library of Tibetan Works and Archives (its publisher) publication page, it does not appear to currently be officially for sale, although there are copies available (both of this 1985 edition and the 1998 version) at a substantial mark-up via the Amazon new/used vendors were one to wish to obtain a copy.