On Amazon it's described by Jacobin (a leftwing magazine in the US popular with Bernie supporters etc): “A searing critique… a necessary contribution to the effort to push contemporary social justice movements further to the left and to weave an understanding of class politics into modern identity-based movements in order to build a radical politics of solidarity.”

All the other summaries on Amazon are similar, i.e. the very top description (New Yorker): "The point of 'Why I Am Not a Feminist' isn’t really that Crispin is not a feminist; it’s that she has no interest in being a part of a club that has opened its doors and lost sight of its politics—a club that would, if she weren’t so busy disavowing it, invite Kellyanne Conway in"

Having never read the book or even heard of the author until you introduced it to me I don't care one way or the other, but it's clear from a reading of these descriptions that her book is a critique from the left.