The Opening of the Wisdom Eye by Tenzin Gyatso, the 14th Dalai Lama, was not what I expected. Not that I had a specific idea of what this was about (I was thinking it might have to do with visualization exercises, the type that are central to Vajrayana practices like the Kalachakra), but I really didn't expect this to be something of a "Tibetan Buddhist primer". It turns out that this is a re-print of a book that H.H. wrote back in 1963 (translated to English in 1966) which was intended as "a clear and comprehensive introduction to the teachings of the Buddha". I'm not sure that the book actually achieves that goal, at least for Western readers, though ... it struck me as more a volume on "how Tibetan Buddhism (and especially Vajrayana) fits into the over-all Buddhist world" as H.H. puts quite a lot of focus into framing Vajrayana as part of a continuum from basic Theraveda practices on through the very complex exercises of his own lineage.
When I say that I don't feel that this book achieves clarity, it's because this is, to a certain extent, a technical manual and it seems to assume a certain level of pre-existing knowledge on the part of the reader. Now, I have read quite a lot of Tibetan Buddhist material over the past few decades, but have not practiced this in any more than the most cursory fashion (I've worked intensely with certain developmental exercises, but have not worked with the material in any systematic form), and there are substantial parts of this book that have me scratching my head and going "oh, uh, yeah ... OK". To randomly pick out an example of this, in the "Training In Supreme Collectedness" section (part of "The Threefold Training" part of the book), there are the Nine States of Mind, the Six Powers, and the Four Mental Activities. Of the latter, #1 is "1. Manonivesapravartak-manaskara - By means of this activity the mind enters into the object." ... that's it, not defining terms, not contextifying action, just the statement as it is. Now, I'm not asking for some Klintonesque "definition of is", but it would be nice to have some commentary about what is meant by "the mind enters into the object"!
However, with this (reasonably serious) caveat, this is a very useful book, because once one gets past (or skims over) the technical jargon, it is, as intended, a very detailed "road map" to what Vajrayana Buddhism is, coming from one whose opinions on the matter could certainly be held to be definitive! As I noted above, this would have been a VERY good book for me to have read prior to my first Kalachakra Initiation (Madison, 1981), as I sort of stumbled into Vajrayana by way of these major public events (I've taken the Kalachakra 3 times, and the Avalokitesvara twice), without much background knowledge other than basic College Religion Major data regarding Buddhism in general.
The Opening of the Wisdom Eye is still in print, so if you were so inclined, you'd likely be able to get it through your local book retailer, but Amazon has it at a very reasonable discount price, and their new/used vendors have "like new" copies under three bucks. Again, this is awfully technical, but is quite a valuable addition to any Buddhist studies collection!