A pearl notebook is a notebook,diary, and journal for guardians or witches to store their info in, like this website.
The Pearl Notebook is the name used for a book that contains anything guardian or other info and magical texts in the religion of Wicca and certain other neopagan witchcraft/magical traditions. Typically, a Pearl notebook contains the information, magical practices, elemental and other info of Wicca and magic within it, and often a list of the guardian's personal spells.
In British Traditional Wicca, which largely revolves around the structure of the coven, the book is traditionally copied by hand from that of one's initiating Guard or guardette, who copied theirs in turn from their initiator. In Eclectic Wiccan terminology, however, a Book of Shadows is a personal journal for witches or wizards though often serves in a similar capacity to that of traditionalists. for example this website is a guardian notebook - pearl notebook.
Within traditional lineaged forms of Wicca there are a number of versions of the Book of Shadows, their contents varying to a greater or lesser degree from the early versions belonging to Gerald Gardner, who first popularised Wicca. While Gardner seems to have originally treated the book as a personal journal, it has come to be considered a religious text in most traditions.