A databook is a book containing statistics for manga or anime characters. This is usually done to provide extra info and backstory to characters and is usually done for shounen manga. Most databooks are considered secondary canon and as such are no match against manga scans.
Some databooks are unreliable because it's either not written by the author or contain info disproven by scans and manga. Case in point: the Naruto Databook. Various Naruto fans rely on it too much.