Before dealing with the book itself, I have to give a shout-out to this amazing series: The Classics of Western Spirituality. If you are serious about studying the great mystics of the West (including those from Christianity, Islam, Judaism, and many less well-known traditions), then you need to read books from this series. Each one begins with extensive introductory remarks on the mystic him/herself, the history of translation for the work, and the history of interpreting the work. Such comment is vital for understanding because mystic texts are often obscure, symbolic, and metaphorical.
This particular text is a good first step in investigating the great mystics of the Christian tradition in their own words. It is relatively easy to follow (although don’t skip the introduction!), and it provides an excellent foundation to two recurring themes in Christian mysticism: (1) Moses as the paradigm of a true mystic and (2) the life of the mystic compared to ascending a mountain. Additionally, it is a wonderful introduction to the allegorical method of interpreting scripture. This method of interpretation was the primary mode for reading the Bible, particularly the Old Testament, for at least the first fifteen hundred years of Christianity (and it still is if you ask any Anglican/Episcopalian, Catholic, or Orthodox Christian).
Also, as I’ve implied or stated elsewhere on this site, to truly explore Christian mysticism you can’t just read about it — you have to practice it. In the same way, while books about particular mystics are helpful, at some point you need to read what they actually wrote — even if it is difficult to understand at first.