It appears to be a collection of extracts from a few Hermetic works (including the Asclepius/Perfect Sermon, the Corpus Hermeticum, and a few Stobaean Fragments) when I flip through a copy of it. It is not a complete translation of these Hermetic works, and what it does contain appears to be primarily dependent on Walter Scott's Hermetica's "translations." (the first three volumes of which are already in the public domain, whose commentary is great but whose "translations" are decidedly not).
Greenlees' book, published in 1949, precedes the publication/translation of the Nag Hammadi Codices, so there's no mention of the Discourse of the Eighth and Ninth, and it also precedes the critical edition of the CH, AH, SH, and other Hermetic fragments put together by A.D. Nock and A.-J. Festugière in the 1960s/1970s (which all subsequent modern translations, like that of Copenhaver or Salaman or Litwa, rely on and which is far more trustworthy than that of Scott). While I am extremely sceptical of its attempts to link the teachings of Hermeticism in a superficial way to Christianity, Islam, Confucianism, Zoroastrianism, and other religions, this was a fairly common Theosophical thing to do, and this book is a publication of the Theosophical Society, after all.
The book does, however, offer some really useful comments, summaries, a well-written brief list of technical words used in Hermetic discourse (such as nous, logos, and gnosis), and even a sort of mini-catechism and FAQ of its own to a number of aspects of Hermetic doctrine. I would disagree on a few points here and there, and at times the scholarship and commentary in the book can be a bit dated and stilted towards a Christian or Gnostic viewpoint that more modern scholars and practitioners have generally distanced themselves from, but I think it's actually really neat to read and makes some good points of its own in a fairly accessible way.