Old English boc "book, writing, written document," traditionally from Proto-Germanic *bokiz "beech" (cf. German Buch "book" Buche "beech;" see beech), the notion being of beechwood tablets on which runes were inscribed, but it may be from the tree itself (people still carve initials in them). The Old English word originally meant any written document. Latin and Sanskrit also have words for "writing" that are based on tree names ("birch" and "ash," respectively). Meaning "libretto of an opera" is from 1768. A betting book is from 1856.
