The book contains a dozen or so "stories" supposedly based on events in Castaneda's life, which all lead back to the concept from the title, that of "infinity", one of those typically slippery terms in his books which gets presented, but not necessarily nailed down with a "normal perception" definition. Don Juan tells him: "Sorcerers have only one point of reference: infinity.", and that he must disengage from all his mundane points of reference. The stories here are about events which brought the author (from a young lad right up through his later years) in touch with "infinity". As to what this means, the closest that I can come (having not studied with any "Toltec" teachers) to what I believe is being meant with this is the sense of gut-twisting vertigo that one gets when, say, contemplating the expansion of the universe (or maybe that's just me).
Anyway, each chapter (which build up into various thematic sections) focuses on one story, and Don Juan's take on what realization that Castaneda should have from that. As with most of his books, there are new concepts introduced in The Active Side of Infinity, the most fascinating being a type of "inorganic being" called a "flyer" which the sorcerers "see" as a leaping shadow. These beings, according to Don Juan, attached themselves to humanity a few thousand years ago, and feed off of us (consuming parts of our energy fields, the "glowing coat of awareness"), creating the "normal reality" via the "the foreign installation" to keep us controlled, something along the ways that people keep chickens in a coop. These beings were not always attached to us, and before that humans were "complete beings" and able to do various mythic/heroic things. The "flyers" can be stymied, however, via what Don Juan describes as "discipline" (a way of engaging "infinity") which ends up effecting the energies of our "glowing coat of awareness" and making that unpalatable as a food source; once they no longer are "pruning" it, this begins to grow back to its natural state (hence the seeming "amazing powers" wielded by the sorcerers).
Needless to say, "Your Mileage May Vary" when dealing with this sort of esoterica. Having at this point read pretty much all of Castaneda's books, this fits in as well as any of the "out there" concepts (like "The Eagle" consuming one's energy upon death if one hasn't learned to slip by it, as described in The Eagle's Gift) that he's presented, so it "fits" in the over-all development of the "Toltec" mythos. I must admit, however, there were several points in the book where I just plain didn't believe his stories, not for over-the-top metaphysical wookie-wookie, but for (to pick one example) that he was a pool shark at age 9 or so, sneaking out of the house at night to work for professional gamblers (billiards never, to my recall, having been mentioned in any of the previous books!).
This said, it was a fairly engrossing read, and one I'd certainly recommend to anybody "into this stuff", although hardly as a starting place for those not familiar with the Castaneda material! The Active Side of Infinity is in print in a paperback edition (so should be easy enough to find), although I got a "very good" (which ended up being indistinguishable from "new") copy of the hardcover for under nine bucks with shipping from the Amazon new/used vendors.