Books of authority is a term used by legal writers to refer to a number of early legal textbooks that are excepted from the rule that textbooks (and all books other than statute or law report) are not treated as authorities by the courts of England and Wales and other common law jurisdictions.
The primary reason for this practice is the difficulty associated with ascertaining the law of the medieval and early modern periods.
On the subject of this practice, William Blackstone said: Besides these reporters, there are also other authors, to whom great veneration and respect is paid by the students of the common law.
Such are Glanvill and Bracton, Britton and Fleta, Hengham and Littleton, Statham, Brooke, Fitzherbert and Staundeforde, with some others of antient date; whose treatises are cited as authority, and are evidence that cases have formerly happened, in which such and such points were determined, which are now become settled and first principles.
[4] More recent scholarly analysis, however, reveals that only the Parva (a set of lectures directed towards junior-level law students) is conclusively his.