Definition
Ligature
A ligature is a single glyph replacing two or more letters that would otherwise collide — most commonly fi, fl, ff, ffi and ffl. Good book typography leaves standard ligatures on.
The classic case is fi: in most serif faces the f's hood runs into the i's dot, so the typeface supplies one drawn glyph for the pair. Standard ligatures should stay enabled in print. Discretionary ligatures — st, ct and similar historical flourishes — are a deliberate period effect and are wrong in a contemporary novel. One practical warning: some ereaders and PDF text-extraction tools index a ligature as a single character, which is why searching a PDF for "find" occasionally fails.
See it handled in your own manuscript
Ceeryo sets your .docx as a book — justified and hyphenated text with widows, orphans and rivers controlled, drop caps, running heads — and shows you every spread before you pay anything.