What is Books?

Legal Definition
A collection of the financial records of a business.

In the investment sense, a book is a record of all the positions (e.g., long and short) that an investor has taken.
Legal Definition
Commerce, accounts. Merchants, traders, and other persons, who are desirous of understanding their affairs, and of explaining them when necessary, keep, 1. a day book; 2. a journal; 3. a ledger; 4. a letter book; 5. an invoice book; 6. a cash book; 7. a bill book; 8. a bank book; and 9. a cheek book. The reader is referred to these several articles. Commercial books are kept by single or by double entry.
-- Bouviers Law Dictionary
Legal Definition
Ail the volumes which contnin authentic reports of decisions in English courts, from the earliest times to the present, are called, par excellence, "The Books." Wharton.
-- Black's Law Dictionary