Whether you are interested in learning more about the Montessori Method itself or how to use it with your child, I suggest The Essential Montessori by Elizabeth G. Hainstock. She gives a great overview of Montessori and then she explains a bit about how to use it at home.
Hainstock's book also gives good quotes from Maria Montessori's own writings such as The Absorbent Mind, The Montessori Method, The Discovery of the Child, The Secret of Childhood, From Childhood to Adolescence, Dr. Montessori's Own Handbook, Spontaneous Activity in Education, and Childhood Education.
The Celebration of Women Writers project provides The Montessori Method online for no charge and here is the link:
The Montessori Method can be a bit hard to read, but we highly suggest at least skimming over it and looking for good quotes because it really does give you a flavor of the original Montessori idea.
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