"She can't even chew gum and walk in a straight line, let alone write a book"

In this quote, Liam Gallagher is utilizing a funny and exaggerated contrast to communicate his opinion that the person in question is not efficient in completing a simple task, not to mention a more complicated one. By saying "she can't even chew gum and walk in a straight line," Gallagher is implying that the individual is so uncoordinated and doing not have in basic motor abilities that they battle with even one of the most standard actions. The addition of "let alone compose a book" even more highlights the person's incompetence, as composing a book requires a considerable amount of ability and effort. Overall, Gallagher is utilizing this quote to mock and belittle the person's capabilities.