Think about it ... how can you sculpt a human if you don't know what to sculpt? The surface contours of a human or animal are defined by the underlying structures, the bone and muscles. Hence you need to study those structures so you know how to actually sculpt or model a human or animal. The following are books on Amazon on anatomy for artists:

Anatomy for Sculptors: Understanding the Human Figure

Anatomy for 3D Artists: The Essential Guide for CG Professionals

Basic Human Anatomy: An Essential Visual Guide for Artists

Classic Human Anatomy: The Artist's Guide to Form, Function, and Movement

An Atlas of Animal Anatomy for Artists

Or you can use Makehuman.