Robert Bringhurst, The Elements of Typographic Style (p. 98):
There are typographers who resolutely avoid using any typeface designed in an earlier era, but even they must learn something of how the older letterforms functioned, because the ancient forms are living in the new. Typographers who willingly use the old faces, and who wish to use them intelligently, need to know all they can learn about the heritage they enjoy.
For learning about that heritage, Elements is a great place to start. See also Alexander Lawson’s Anatomy of a Typeface, which goes into more detail about specific typefaces, and Stephen Coles' The Anatomy of Type, which is more visual and more recent.