Jan Tschichold on Good Lettering
Good lettering demands three things: — (1) Good letters. A beautiful letter form must be selected which is appropriate to the purpose it is to serve and to the lettering technique to be used. — (2) Good design in all details. This calls for well balanced and sensitive letter spacing and word spacing which takes the letter spacing into account. — (3) A good layout. An harmonious and logical arrangement of lines is essential.
The intro of Jan Tschichold’s Treasury of Alphabets and Lettering has useful guides on letter spacing of capitals and lowercases.