Pocketbook is another name for a purse or wallet, a place to keep money or other essentials. Pocketbook can also refer to the amount of money you have, even if it’s in your mattress.
In the UK, pocketbook is a pocket-sized notebook, but in the US it's usually a small bag typically carried by women. Figuratively, your pocketbook is your budget: "Those shoes are way too expensive for my pocketbook." In the early 1600s, a pocketbook was "a book small enough to fit in a pocket," and by 1722 it became "a booklike leather folder for papers." In the early 1800s, it came to mean "woman's handbag."