This was the first Montessori book I read and would recommend it as a good starting point for those new to Montessori. It is a short easy read that is not overloaded with too much Montessori information which can sometimes be overwhelming.
Montessori Play and Learn: A Parents' Guide to Purposeful Play from Two to Six, by Lesley Britton
Also a good introduction to the Montessori Method. The first half of the book would actually be good reading for all parents in general. It covers the importance of the role of the parent in the development of their children. It also offers good suggestions on how to make your home more child-friendly.
Montessori: A Modern Approach, by Paula Polk Lillard
A good book if you are interested in more in depth reading of the Montessori approach. Not technically ‘modern’ since it was written many years ago. However, the Montessori method hasn’t changed since, so this book still applies today.
By the way, I found all of these books at our local library. I would suggest visiting a library and browsing through their collection of books in the subject. The library has been a great resource for me. Something I recently learned is that if my local library doesn’t have a book I need, I can request it and they will borrow a copy from another county library. I then get an email when the book arrives. How convenient!