This book, long out of print, is extremely difficult to find; prices on the secondhand market are usually well in excess of $1,000. I didn't pay that much for it -- after years of looking, I found someone who seemed to not quite know what they had, and was selling it at a serious discount -- but it was still pretty expensive.
But if you're interested in outsider art in general -- and especially if you're interested in Darger in particular -- eventually, you're going to have to buy a copy, or track it down at a library or something. In a real way, MacGregor is the wellspring of Darger scholarship; though he wasn't the first to publish a monograph about Darger, this gigantic doorstop of a volume -- it's more than 700 pages long -- is the most detailed. Any attempt to wrestle with Darger's work eventually finds itself in dialogue with MacGregor.
The book itself is beautifully presented and lavishly illustrated -- many of these works are ones I haven't seen anywhere else, and I own most of the useful books ever written about Darger. It's especially useful for its reproduction of Darger's most disturbingly violent art, which is often mentioned in connection with the artist, but rarely seen.
I've refrained from rating the book because I'm not sure exactly how to do so. On the one hand, MacGregor probably spent more concentrated time with Darger's work than any human being other than the man himself. His descriptions of Darger's working methods are brilliant and valuable, and the almost unimaginable amount of research that he did on the circumstances of Darger's life pays immense dividends. The book is extremely useful just for those aspects.
The downside -- and it's a major one -- is that MacGregor is an unrepentant, unreconstructed Freudian, and views all of Darger's work through that lens. Freud is, of course, critically important from a historical standpoint, but very few mental health clinicians of any kind stand by his theories today. But MacGregor has no compunctions about trying to psychoanalyze Darger, and I think it leads him astray repeatedly. The entire section in Chapter 11 where MacGregor compares Darger's work to, for instance, Jeffrey Dahmer's altar, is both embarrassing and unconvincing, and it's just the worst example of many.
And there are many opportunities, because compared to Darger, Salvador Dali and Georgia O'Keefe might as well be Mondrian; I can't think of another artist who offers a more tempting target for a Freudian than Darger, with his massive depictions of armies of nude prepubescent girls with penises. But these temptations do MacGregor no favors. He dismisses, for example, that the hyper-violent, sadomasochistic language of Catholic martyrologies is a meaningful source for Darger's own depictions of butchery, a denial that Moon takes out behind the woodshed in DARGER'S RESOURCES. And Appendix A, where MacGregor diagnoses Darger as having what was then called Asperger's, reads like something from the Paleolithic compared to current understandings of the autism spectrum. (My stepdaughter has what would have been listed as Asperger's, and I barely recognize her in the descriptions that MacGregor gives.)
Be that as it may, I'm extremely glad to have (finally!) read this book; Darger is one of the artists most personally meaningful to me, and there's enough insider information and beautifully-reproduced art to be worth what I paid for it. Just know that the book also has real flaws, flaws which many later analyses have started to correct.