Perhaps the key of the attraction of this book is that the author appears to have been somebody who had encountered (and independently studied) the work of Gurdjieff and not a "follower" of any of the various 4th Way lineages, and had embarked on this book as a way of clarifying the man and his teachings primarily for his own personal edification ... a stance that I can, obviously, relate to, having read a couple of dozen books by or about G.I. Gurdjieff and P.D. Ouspensky, but never having any encounter with their surviving schools.
Shirley's Gurdjieff is primarily a history, taking a "non-mythologized" look at the where/what/when of Gurdjieff's life, something which is difficult to track via the canonical materials. The author peels back the veils and tries to place the biographical bits into a literal context of the times, from the influences on the young Gurdjieff, to his "search for Truth" and his inner circle, and how that ebbed and flowed through the chaos of the Russian Revolution, and into the start of his "Institute for the Harmonious Development of Man", his interfacing with then alienating Ouspensky, and the eventual production of the books which put forth the "Fourth Way" teachings. As noted, I've read a goodly amount on the subject, but this book puts the whole "Gurdjieff group" in clearer context than I can recall seeing it, with a clear sense of who did what, where, and when (with juicy tidbits like when he told Aleister Crowley to "go and never come back" when the famed occultist made an uninvited visit to the Institute in 1926).
One of the most "useful" bits here is Shirley's "wiseacre interpretation" of the very difficult Beelzebub's Tales To His Grandson, taking the tortured (albeit carefully crafted) prose and wringing out from it some core ideas. Gurdjieff was fond of inventing ridiculously convoluted names for things discussed in Beelzebub's, and of endlessly on-running sentences. A.R. Orage spent years working with Gurdjieff to wrest an English version out of the the Russian and Armenian manuscript, and Shirley's glosses on this are very welcome (I wish he'd do a interpretation of the whole 3-volume 1,200-page book!).
Also useful is the appendix which discusses "Bibliographical Suggestions" both of the Gurdjieff/Ouspensky canon (these will be forever linked by Ouspensky's In Search of the Miraculous which expresses Gurdjieff's teachings in a way that eventually healed the rift between them), and of books by and about their various followers.
I was reminded in reading bits of this (including another appendix about the "food for the moon" concept) of how some of Carlos Castaneda's critics have claimed that much of Castaneda's "system" was in many ways cribbed from Gurdjieff's ... the latter put out as plainly as Shirley does here allows for those very interesting parallels to be more evident than they might otherwise appear!
Needless to say, I highly recommend Gurdjieff: An Introduction to His Life and Ideas ... and it does still appear to be in print, so could be found at your local brick-and-mortar bookseller. However, Amazon has it at 25% off of cover, and you can get a "new" copy from the new/used vendors for under five bucks (plus shipping). If you have any interest of learning about Gurdjieff and his teachings (or wouldn't mind having a fresh look), do pick up a copy of this!