The Cactaceae is comprised of 4 volumes about cacti. It was written by Nathaniel Lord Britton and Joseph Nelson Rose and published between 1919 and 1923. It was published through the Carnegie Institution. It was a monumental undertaking that extensively reorganized cactus taxonomy and that is still considered a cornerstone of the field of cactus grouping, classifying, and naming. It contains multiple black and white photographs and many fine color plates by Mary Emily Eaton, a talented botanical artist.
Prior to the publication of The Cactaceae, there was no comprehensive approach to the treatment of cacti, and chaos enveloped the world of cactus taxonomy. It was almost impossible to study the family because the names of plants were too often nonsensical. A few (about 30) large supergenera genera were recognized that did little to organize the cacti into sensible groups. The Cactaceae refined 20-30 supergenera into over 120 sensible genera. The creation of the new genera followed typical botanical practices and used flower and fruit characteristics as well as vegetative and geographic considerations (modern approaches).
The first volume, published in 1919, addressed the Opuntiae Tribe. Due to my interests, this is my favorite volume. Volumes II, III, and IV addressed the of the Cereae Tribe (all the other cacti of the time). The Cactaceae divided the two tribes into sensible genera, many of which are still used today.
Additional Reading: The Cactaceae, v3