Why!? It's more money and more time reading - I've read all the Montessori books....
But those Montessori books aren't going to answer all your questions. The Theory papers (link goes to the elementary theory album - primary is also available on that same site) put all the information into particular headings, and provide modern-day insights that are not only useful, but absolutely necessary to "get" Montessori. Most of the "how" questions are answered in theory - as well as the environmental, and the why questions, and the overview for each area and how each area fits with the others....
|doing a puzzle without a frame|
(tessellations, that is the point;
Montessori details without the "big picture"
- not so much)
You'll understand how a Montessori classroom works without ever touching an "official" Montessori material.
You'll understand how to "Montessorize" subjects from non-Montessori sources (ie when you don't have a choice; or the Montessori option is unattainable for you at this time).
If you don't have the theory, you're missing out on the fullness of cosmic education - something that can be done without one single Montessori material; but cannot be done unless you know about it :)
And if you do have the theory and NO OTHER ALBUMS, you'll have the environment and the atmosphere down.
Keys of the World has theory for primary and elementary
(different links from above - same theory albums)
Please respond in comments with other theory albums that are available at other places.