Five Simple Steps to Better Typography: Ligatures
http://www.markboulton.co.uk/journal/comments/five_simple_steps_to_better_typography_part_3/

Boulton, Mark
Mark Boulton
2005

Abstract:

The third installment of this series is dedicated to just one typographic element - Ligatures. Ligatures are combinations of letters - some of them are functional, some are decorative. They are more commonly seen in serif faces, although ligatures in sans-serif faces such as Gill Sans and Scala Sans are important to the typeface and should be used.