A key aspect to the preservation and transmission of the Buddhas teachings lies in the way that they are passed on from generation to generation. There are certain teachings that anyone can read or practice.
Choose a book, read it, reflect on it, use it to support your practice.
There are other texts that traditionally require a lung (pronounced with long u). Lung is the Tibetan for oral or reading transmission. It is an authorization to study or practice a text, but also more than that. In order to receive the lung for a text, you need to get it from someone who already has the lung. So the lung is a living transmission from teacher to student from generation to generation. The teachings are alive because the lung is intact.
The key aspect of maintaining the living tradition of the Buddha's teachings is the teacher-student connection.
This also goes much deeper. There are hundreds of teachings of the Buddha and other great masters that no longer have a living lineage of transmission. You could find one of these texts, have it translated and read it. It might be an amazing teaching that profoundly impacts your life, but who do you talk to about it? Have you ever read a book that makes a huge impact on your life or perspective? You want to share it with someone, you want to talk about it, discuss challenging sections.
The tradition of passing on the teachings through an oral tradition from teacher to student ensures that the student has someone to come back to with questions. They have a network they can tap into if they need it.
If you could only read books if you received the oral transmission, what do you think you would receive? If you gave out oral transmissions of books that made an impact in your life, which ones would you want to pass on (assuming you had received the lung to begin with!)?
If we only read books due to connections we had with others, would you be advocating for giving as many lungs as possible?