Despite its merits, I am afraid the book leaves us with a deficient understanding of the logic of such arguments and with no practical proposal for solving at least a broad range of such cases.
In particular, Walton does not cope with gradualism as a plausible ontological alternative. He tries to implement a merely pragmatic approach to the treatment of slippery slope arguments. But a good account of such arguments cannot be only pragmatic -- a logical assessment of the validity of the arguments is also called for. On the other hand, a satisfactory pragmatic approach has to explain that vagueness is a matter of use, not of meaning.