Dr. Maria Montessori developed an approach to working with children based on observation and response - through observation, she found the best response was to provide key experiences based on real life: keys which open the doors for discovery and exploration.
The keys she developed are found in albums and training (albums being the written description of the key experiences as provided in training) which adhere to the concept of the "universal child" and leave time and psychological space for exploration and personal discovery.
Yes there are some albums/trainings that have too much, modified much over time, with many details that Montessori left for exploration and personal discovery now handed to the child. There are still VERY good presentations and suggestions, just perhaps beyond the keys.
The adult can only know how to utilize the materials through the use of a good set of albums that guides, but does not dictate the scope and sequence of the child's exploration. Albums contain the keys. If albums were useless, teachers in training would not have to create them ;) Teachers in training receive presentations from the trainer, then practice with the materials, then write-up the descriptions of how to use those materials, with pre-requisite suggestions, main purpose (direct aim), follow-up purposes (indirect aims), follow-up activities, and SO MUCH MORE. These albums are then read through by others at least 3 times before the teacher graduates.
Some people are pushing the idea that albums are not only not needed, they are downright anti-Montessori and are a very bad idea. That you can have the materials on the shelves and just explore with them. JUST explore. With no guidance from the adult who is called a "guide"; with no guidance from Maria Montessori beyond the bit she describes the materials in her writings (she provided training and her trainees were the ones to write things down); with no idea what each material does going in. So what is the role of the adult? Set up the materials and walk away? NO! Maria Montessori never said that! The child needs to receive a presentation from a knowledgeable adult (Maria instructed others how to use the materials, said "do it this way with the children, observe, and report back to me the results", then responded accordingly).
Here is the kicker: the albums contain MUCH more than 'how to use the materials' - they are full of key experiences that do NOT utilize a single piece of special material - and many do not use a material at all.
Not everyone needs an album to focus on key real-life experiences that set the child up for exploration and personal discovery with a properly prepared adult who is ready to guide.
And that is great!
Most people DO need a guide-book to guide them in their own journey.