Magick in Theory and Practice is the third part of the work Crowley called Book Four, and it is the operative section: after the parts on mysticism and on the instruments of ritual, this is the part that tells a reader what magick is and how it is performed.
It opens with the definition he is best known for, that magick is the science and art of causing change to occur in conformity with will, and with a set of numbered theorems developed from it. The definition is deliberately wide. Crowley's own footnote gives writing a book and even an ordinary act of will as examples, and the width is the argument: he is claiming that ritual is a special case of intention rather than a separate category of act.
The chapters that follow treat the formulae of ritual, banishing, consecration, invocation, evocation, divination and the eucharist, and they are written as instruction with commentary. The book also contains, in an appendix, the passage about the sacrifice of a male child that Crowley later said referred to something else entirely, and which has followed his reputation ever since.
The atlas describes the work rather than presenting it as instruction.
Book Four as a whole was assembled from material written across two decades and did not appear as a single work in Crowley's lifetime; the four part arrangement most readers use is a later editorial construction.