Both can be easily searched by subject so you can dip in and out as you discover McLuhan's different theories. Literally, Understanding Media is a kit of tools for perception. It is to begin an operation of discovery. It is not the complete work of discovery. It is intended for practical use.
High definition is the state of being well filled with data.
Telephone is a cool medium, or one of low definition, because the ear is given a meagre amount of information. And speech is a cool medium of low definition, because so little is given and so much has to be filled in by the listener. On the other hand, hot media do not leave so much to be filled in or completed by the audience. Hot media are, therefore, low in participation, and cool media are high in participation or completion by the audience. Hear him via his favorite medium, the spoken word, in this two-hour lecture taped in 1978, where McLuhan delivers his message on the media and how they affect our lives.