A Galeotto, indeed, that book, and he who wrote it, too -- A wonderfully subtle (on Dante's part) chiding against interpreting a text before you get to the end.

Speranza

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 first kissed:

“A Galeotto indeed, that book and he / who wrote it, too” 

In the 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.