This is the clearest book I have read, once you get past the dogma. As with all of Kelsang Gyatso's books, you have to accept the author is a traditional religious monk and thus expect there to be conservative religious theology appended to the philosophy being taught. For those who are willing to put this to one side, a great beauty of philosophy of the mind and phenomena awaits.
The book, being an explanation of the classical scripture, the Heart of Wisdom (Prajnaparamita Hridaya) is topped and tailed with such religious elements by beginning with a story of the Buddha on a mountain surrounded not just by monks but gods and spirits. The end of the book instructs the reader on how to engage in meditation and recitation of a mantra to produce magical results such as rolling back the waves - take that King Cnut!
I implore the reader to look past this and focus on the high quality content in the middle as it is an absolute gem. Here you will get what I consider to be the best description of Buddha's most complex teaching, usually dubbed in English as Emptiness, which is essentially the true nature of reality. I would expand on this but believe it is best to leave it up to the book to explain. In addition it will explain the stages that one achieves as they reach the final steps to enlightenment with each one progressively getting more profound in understanding than the last.
Heart of Wisdom is a commentary of the original scripture which appeared late in Buddhist scriptural history. As such, the scripture is part of the Mahayana tradition of Buddhism.
It should be noted that this book is based on a thread of commentaries spanning from the early Nagarjuna (1st century CE), Chandrakirti (7th century) and Tsongkhapa (14th century) as well Kelsang Gyatso's own teacher (2oth century). This may not be obvious to the reader unless you have had extensive academic knowledge of emptiness. This may explain why the explanations are incredibly thorough and clear.
In fact, I would go as far to say that reading this book in my younger years helped to sharpen my mind in being more logical and deductive in my critical thinking. So, there should be benefit even for those who are not of the Mahayana tradition but have an interest in the elusive wisdom teachings. For those who want the full benefit of what this book offers, discussion, contemplation and classes may be required.
TLDR: One of the best explanations on Buddha's most complex teachings; emptiness. For anyone wishing to understand this topic or simply gain a new perspective of mind and reality, I highly recommend reading this; just don't get put off the dogmatic parts at the beginning and end.