This book seems to be a shorter presentation of much of the material found in _The Absorbent Mind_, which requires a good deal more persistence. At less than 100 pages, this book is easy to take in, while still containing fundamental principles of Montessori's theory of education, especially as relevant to 3- to 6-year-olds. The writing is engaging, not academic. I can easily recommend it to be the first or only book by Dr. Montessori for the parent of a toddler to read. There is not much here specifically relevant to the nature of elementary-level Montessori programs, however.
Perhaps the foundational concept of Montessori's overall theory is that of the young child's "absorbent mind": "the child has a type of mind that absorbs knowledge, and thus instructs himself." The universal example is that of language acquisition, which is flawlessly and gleefully accomplished by 2-year-olds who cannot read books or ask questions or attend lectures, while the same entails a very laborious and error-prone effort for adults. The child's mind is, from birth, forming itself and its "organs" (drawing an analogy to embryology) by its own natural impetus and direction. Environment is key. "[T]he child who, making use of all that he finds around him, shapes himself for the future." Up until age 3, the child's mind is "unapproachable by the adult, who can exercise no influence on it." From 3 to 6, the child is able to tirelessly concentrate upon an environment carefully crafted with his or her psyche in mind, absorbing its lessons.
Another fundamental concept in the theory is that of the "normal" child, the "true" child which is revealed only when the environment is right. Defects of character which may have been present before age 3 are resolved during the ages of 3 to 6, if the provided environment plays to the power and nature of the absorbent mind.
The title of the book reflects the author's belief that the world which "has been torn to pieces" (presumably these words were written during or soon after WW2) can only be fundamentally reconstructed through education. Children create themselves, they create adults, and thereby they are the creators of culture and civilization and the whole world -- they are all-powerful in this sense, and therein lies the fundamentality of education as a cause of history.
Montessori also presents a notion of cosmic harmony, such that every organism is performing the role it was perfectly designed, through evolution, to perform. There is both a sense of a normally pleasant universe here, as well as a questionable assumption of some important extra-personal purpose for each of us. Not much in Montessori's program for 3- to 6-year-olds relies on this latter point, that I can find.
Some other points discussed: the importance of using the body, especially the hands, in learning; the stages of physical and mental development, and of language learning; the small child's healthy motivation to complete tasks which may seem pointless to adults; the construction -- not "teaching" -- of moral character; obedience as obtained through freedom, not fear; and what is required of the Montessori teacher or guide.