It depends on what you want to do with the information. If you want to find a montessori school, the websites are really all you need. If you want to TEACH your child montessori ed, then you need books, I'd just use amazon.
The Montessori Method
The Absorbent Mind
Montessori from the Start (infants, toddlers)
Maria Montessori: The Science Behind the Genius (new, describes montessori ed in contrast to trad ed)
A Parent's Guide to the Montessori Classroom
Rita Kramer's biography of Dr. Montessori
Anything written by Aline Wolfe for "spiritual and artistic guidance"
For materials: Montessori Services (look at the pictures, go to restaurant supply stores, the dollar store, and walmart and make your own scrubbing work! Go to michaels and make your own alphabets and math materials)
Furniture, especially for babies, toddlers, preschoolers: Environments, community playthings, hellowood (expensive, but when you compare it to the expensive useless toys people give you, not so expensive, tell the grandparents), ikea for less expensive but less sturdy stuff.