This article is from the Wicca FAQ, by email@example.com (Rain of Teleport) with numerous contributions by others.
The Book of Shadows (or "BoS") is sort of a customized reference book
for Wiccans, containing useful information such as myths, liturgical
items, one's own writings or records of dreams and magical workings.
According to Gerald Gardner, such a book should be handcopied from teacher
to student but in practice not every Wiccan has a "book of shadows" and
few are exactly alike. Sometimes only initiates are allowed access to a
tradition's book, or it may be called by a different name, such as "mirror
book," "magical diary" or "grimoire." There are many "books of shadows"
available in print and on-line (leading to the "disk of shadows" or even
"directories of shadows" several megabytes large). If you'd like to copy
from these sources for your personal use, you may assemble your own book,
but please observe copyright laws in your newfound enthusiasm.