These are really useful, simple to understand diagrams to see the important aspects of type, and the rules and guide lines in which to produce good typography. I new the basic rules of good type layout but always forgot what the different parts of letters were called, which is really important if you are going to take it seriously.
Also in these diagrams they highlight the sections they talk about which again makes the whole thing really easy to understand.
Books used : Type-One - Discipline and progress in typography.