I don't know what book the OP was referring to, but a book that absolutely needed multiple fonts is "The Interior Life" by Dorothy Heydt (writing as Katherine Blake). The switch back-and-forth between "exterior" worlds and "interior" one needs a very strong visual indicator to work. As the author said (in 2016):
The Interior Life uses three different fonts, to represent the three "worlds" in which it takes place. The original Baen publication used fonts that probably looked fine to the typesetter, but were indistinguishable to the average reader. The online editions now display three fonts that are easier to tell apart. Which version you download will depend on whether your e-reader is a Kindle or not.
(
https://www.kithrup.com/~djheydt/)