Learning witchcraft is more a process than simply a "how-to" instructional, and to begin one must find a teacher or some resource from which they can learn. Many places have local witches who can be sought out via various means and libraries contain resource books on the subject.
There are many variations on witchcraft. Some involve Wicca, a religion that was brought to public light during the 1950s. It embraces witchcraft and follows a path outlined by a man named Gerald Gardner. Wicca is not the only path to witchcraft, however, and not all witches are Wiccan.
Many practicing witches or aspiring witches keep a personal Book of Shadows, which is often confused with a spell book. A Book of Shadows can contain spells as well, but it also contains your own personal experiences with witchcraft, your learning process and any experimentation you may do while learning or in the future. It is more like a journal than just a spell book, but it's also a good place to keep spells you may want to use frequently but are too long to memorize.
There are a lot of branches of Wicca and to begin with, you should find a local coven, group or teacher in order to begin learning witchcraft.