I know it has been a few weeks since you posted your question, but I hope this info is still of interest. I attended Montessori schools from the age of 2 until 11. My son has attended since he was 21 months, and we are now using Montessori principles to homeschool.
There is a huge amount of information that the Montessori method is based on. I will try to quickly cover some of the basics.
Maria Montessori (who was the first woman to receive a M.D. degree in Italy) spent many years observing children before developing her system of education. One of the most basic tenants of Montessori is that by providing an ordered environment, adults can free the child to concentrate on their inner needs. The Montessori classroom is designed with the assumption that children have a natural hunger for knowledge and "work."
Sometimes Montessori is described as being very rigid, sometimes it is described as being very free. In truth, a well run Montessori classroom is very rigid in its design and by being so frees the child.
Like LaLa said, the teacher acts as a guide more than a teacher. All of the instuction is done using hands on material, which should be made of wood, nice fabrics, metals, etc. All concepts are presented at a very concrete level, and then later on at an abstract level. One part of Montessori which is rigid is that materials are presented in a very specific order to each child. (Different children receive the lessons at different times depending on their interest and abilities, but always in the same order within each area of work, i.e. the sandpaper letters are introduced and then when the child has mastered all of the sounds of the letters and has started blending sounds theyare introduced to the moveable alphabet materials) The thing about this is that Montessori observed that sometimes children need to go back to materials that they have already mastered, either because they have spent a lot of energy learning a new concept, and old materials provide a nice break, or because they need to review an old concept, etc. this is one reason for the multi age classrooms. By being free to choose from any materials that the child has been introduced to they are able to balance their own need to learn new concepts and review existing concepts.
Also, all of the materials are designed to provide children with information that corresponds to what Montessori called sensitive periods when the child is particularly open to the information. Sort of along the lines of all the info out there about foreign language being easier for small children. A lot of the sensorial materials deal with this, weight, color gradation, temperature differences, smells, etc...
Anyways, if you can't tell I am a huge believer in the Montessori method. I think that when well implemented her ideas truly reflect the needs of children, not an interpretation by adults of how and what children should need. There are some great books out there, although I would recommend reading about Montessori before trying to read Montessoris actual works. Her writing style is very technical. Not impossible to read, but requires a lot of attention. Hope this helps...