meaning - Why is a woman's purse called a "pocketbook"?

It's not a book, and it doesn't fit in anyone's pocket. Why does my brother-in-law insist on calling his wife's purse a pocketbook?

I'm interested in the etymology, and in the chronological and geographical distribution of this expression. My sister & I (raised in southern California, can't get used to such an obvious misnomer) vs. bro-in-law (raised in New England, doesn't understand why we're bothered) are only one data point, and only on the geographical scale.

Answer

From a blog entry at Separated by a Common Language, I learned that Luanne von Schneidemesser wrote about the word purse and its synonyms in a 1980 piece for American Speech. The article is dated, but addresses part of your question directly.

Her etymology of pocketbook mirrors answers given here already:

Pocketbook was originally just that: a small book that could be carried in the pocket. The OED shows that by 1685 it was understood also to be a "book for notes, memoranda, etc., intended to be carried in the pocket; a notebook; also, a book-like case of leather or the like, having compartments for papers, bank-notes, bills, etc." In the last meaning the DAE attests its use in the United States since 1816.

She then analyzes responses given from a 1000-person sample of Americans to the question, "What do you keep money in when you carry it around with you?", posed in a survey done for the Dictionary of American Regional English.

After a discussion of the distribution of wallet and billfold, she addresses purse, pocketbook, and handbag:

Purse, pocketbook, and handbag are all standard terms reported from all parts of the country. Yet purse, according to DARE's 569 responses, is not quite as frequent in the Northeast and coastal Atlantic states as it is to the west of those areas. In the eastern areas, pocketbook (395 responses) appears about as often as purse; but farther west, it becomes sparser. Both purse and pocketbook are distributed by age, community type, race, and education in correspondence with the total DARE sample.

This last point seems to rule out any significant generational differences among pocketbook respondents—30 years ago, that is.

Schneidemesser cites two other surveys in her piece. In one done for the Linguistic Atlas of New England, she notes a size distinction between purse and pocketbook with the former being considered a "small pouch or similar container for coins or other money," and the latter a "larger receptacle used to contain paper money as well as other articles," including a purse. In the second, for Elizabeth Bright's Word Geography of California and Nevada, she shows only 16 percent of respondents used the term pocketbook.

^{Purse and Its Synonyms, American Speech, Vol. 55, No. 1 [Spring, 1980], pp. 74-76}

Edit 3/4/11:

An informal survey of five female family members (lifelong New Englanders) shows there may be some generational shift after all:

great-grandmother: pocketbook (reportedly)
grandmother: pocketbook/bag
mother: pocketbook/purse
sister: purse
niece: purse