In the book, amphiboly is defined as "when
an ambiguous statement serves as a premise with the
interpretation that makes it true and a conclusion is
drawn from it on an interpretation that makes the
The fallacy always applies to an argument. An ambiguous statement is not amphiboly. There has to be a conclusion resulting from the ambiguous statement.
Under this definition, "being perfectly frank" is the ambiguous statement as @causative answered.
If "being perfectly frank" refers to "you", it then serves as a premise, and "you should probably lie ..." is the conclusion.
The conclusion, of course, contradicts the premise and thus drawn from an interpretation that makes the premise false (i.e. "I" am being frank, not "you").
You can compare this to the bigamy example given in the book as well.