In Canto V, Francesca, is describing how she and Paolo Malatesta, her adulterous lover, fell in
love, blames the book and author of the book that she was reading when they
“A Galeotto indeed, that book and he / who wrote it, too”
version of the poem of "Lancelot" that Francesca is reading, Galeotto, serves as an intermediary between
Lancelot and 'la regina Ginebra'.
Francesca slides the blame for her kissing Paolo and entering into
adultery onto the book and its author.
However, if she’d gotten slightly farther, as
opposed to getting distracted by kissing, she’d have gotten to the bit which
basically said, adultery, a bad idea, don’t do it.
Thus, Francesca’s excuse for adultery -- "the book it
made me do it" -- is a wonderfully subtle (on Dante’s part) chiding against
interpreting a text before you get to the end. Or for that matter blaming an
author for your sins.