pocketbook n.

mid-14c., pokete, "small bag or pouch, small sack," from Anglo-French pokete (13c.), diminutive of Old North French poque "bag" (Old French pouche), from a Germanic source akin to Frankish *pokka "bag," from Proto-Germanic *puk- (see poke (n.1)).

The narrower meaning "small bag worn on the person, especially one sewn into a garment" is from early 15c. The sense of "one of the small bags or nets at the corners and sides of some billiards tables" is from 1754. The mining sense of "cavity in the ground filled with ore" is attested from 1850; the military sense of "area held by troops almost surrounded by the enemy" is from 1918; the general sense of "small area different than its surroundings" (1926) apparently was extended from the military use.

Figuratively, "one's money" (conceived as being kept in a pocket), from 1717; hence to be out of pocket "expend or lose money" (1690s); Pope Pokett (late 15c.) was figurative of the greedy and corrupt Church.

Middle English bok, from Old English boc "book, writing, written document," generally referred (despite phonetic difficulties) to Proto-Germanic *bōk(ō)-, from *bokiz "beech" (source also of 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).

Latin and Sanskrit also have words for "writing" that are based on tree names ("birch" and "ash," respectively). And compare French livre "book," from Latin librum, originally "the inner bark of trees" (see library).

The sense gradually narrowed by early Middle English to "a written work covering many pages fastened together and bound," also "a literary composition" in any form, of however many volumes. Later also "bound pages," whether written on or not. In 19c. it also could mean "a magazine;" in 20c., a telephone directory.

The use of books or written charters was introduced in Anglo-Saxon times by the ecclesiastics, as affording more permanent and satisfactory evidence of a grant or conveyance of land than the symbolical or actual delivery of possession before witnesses, which was the method then in vogue. [Century Dictionary] 

From c. 1200 as "a main subdivision of a larger work." The meaning "libretto of an opera" is from 1768. A betting book "record of bets made" is from 1812. The meaning "sum of criminal charges" is from 1926, hence slang phrase throw the book at (1932). Book of Life "the roll of those chosen for eternal life" is from mid-14c. Book of the month is from 1926. To do something by the book "according to the rules" is from 1590s.