Glenn H. Mullin's The Practice of Kalachakra was published in '91, and is a solid over-view of the Kalachakra, both putting it in context of history and the Vajrayana Buddhist tradition, and supplying key historical texts. Mullin is a prolific translator/interpreter of Tibetan texts, and is affiliated with the Library of Tibetan Works and Archives, and in this volume he does both, the first half being an exposition of the Kalachakra's history, and the second being eight texts from leading Lamas, including several of the incarnations of the Dalai Lama (the 1st, 7th, 13th, and the current 14th).
While this book is not "restricted", I still find it hard to cogently write something about the content per se, simply because of the complexity of the subject. Where does one start? Buddhism is an evolving faith, with aspects that are strongly counter-intuitive to the Western mind (for instance, within the Kalachakra materials it is given that the historical Buddha taught the Kalachakra at the request of the King of Shambhala somewhere around 500bce, but the teachings were only preserved in Shambhala for the first thousand years after the time of the Buddha, and didn't reappear in the mundane world in the first millennium of the common era). Plus, from the Tibetan view, there are progressives layers of "paths", with the Sutrayana, Mantrayana, Tanrayana, and Vajrayana (expanding on the usual Hinayana/Mahayana division), each becoming more technical, but including the previous approaches.
As I've noted in previous reviews on the subject, the Kalachakra is unusual in that, while being from the highest levels of Vajrayana practices, it has been (at least in recent times) generally presented in large gatherings where anybody can attend. The reason for this has been explained that for these Tantric practices "to work", one must be given ritual permissions, and there have been countless beings who have practiced the Kalachakra over the centuries, but very few who were able to make the difficult journey to a major monastic center to actually go through the initiations. By allowing any and all to attend (and some of these in India have had a quarter million people show up!) it is felt that those who have karmic ties to the Kalachakra can, retroactively, get the necessary permissions, hopefully leading to enlightenment.
Speaking of enlightenment, it has occurred to me that what appeals to many Westerners about the Kalachakra, is that it is (within its context) the "quick path". While other paths might hold that practice over numerous lifetimes may bring one to a rebirth in a favorable setting for enlightenment, the Kalachakra claims to provide a way to full enlightenment in this lifetime, and even within a "doable" window of time within one's current lifetime. Not that this is an "easy way", as the effective practice of Kalachakra requires one to encompass virtually all of the regimens and prescriptions of the preceding paths, and it necessitates a level of mental concentration that is not easily attained. However, I do think that the prospect that "if I really work intensely at this for the next 10-15 years I'll achieve this" is quite alluring to those of us in the West for whom the concept of cyclic rebirths is somewhat alien!
Anyway, if this sounds interesting to you (karmic connections?), you may want to check out The Practice of Kalachakra, as it will certainly give you a good basis in what the Kalachakra is about. This is still in print, but there don't seem to be a lot of "deals" available for it, so your best bet is probably a sale off of retail (like one of those Borders or B&N coupons for 20% off a single item)!