Book of Shadows
In Wicca there is no so called holy book or sacred text like the other religions of the world have. There are some texts and some traditions which are regarded as very important, and it influences the beliefs and practices of the people of the sect. Book of shadows is one such book for Wiccans, which influences the beliefs and rituals of the Wiccan people. These books have excerpts from various other books, and possess some poems and chats, composed by the high priests and high priestess over a long period of time. Not only that, the book also has rules which will guide a Wiccan to perform spells and rituals.
Though the name of the book is same, there are variations of the book in different sects. The difference is between the spells and the excerpts that are there in the book. Sometimes a little or huge part of the book has been altered based on the fact that some of the spells turned out to be not effective. The requirement for such a books was always there because in different period of time people felt like writing down some of the nuanced aspects of the practices and rituals observed by the sect. as there is no such written and proved version of the book, loads of Wiccans do not believe in the book itself.
Like other rule books of other religions, book of shadows is not rigid. Loads of changes happened to it over the years and more changes are expected to happen. In some of the traditions the rituals and practices were written down and that is why the requirement for the book was felt. In other traditions the rituals were never written about and that is why there was no requirement for the book at all. In Wicca the solitary Wiccans are also allowed to make changes to the book. They have made loads of changes over the year depending on their requirements and that is why, you will get as many version of the book as you want.
In any other religion you will not find this. The rule books or books that have all the information about the rituals and practice written on them are far more rigid than the book of shadows. Once something is written, no changes are done. If some changes are brought, they are brought after many discussions and with utter delicacy, so that the importance and the significance of the book are never lost.