John Castaing’s 'An interest-book at 4, 5, 6, 7, 8 per c. from 1000 l. to 1 l., for 1 day to 92 days, and for 3, 6, 9, 12 months' was published in 1700. It sets out tables allowing the user to look up the interest gained on different sums of money over various time periods, rather than having to calculate it. Because it was used as a practical item in daily life, and therefore handled roughly and regularly, few copies have survived. There are three others in the UK, and one in the US.
Interest is the amount a lender charges for lending out money, usually expressed as a percentage of the loan. Determining interest was crucial to many of the financial and economic innovations of the Financial Revolution, which occurred during the late 1600s. Although earlier textbooks explained how to calculate it, handy reference books that eliminated the need for any sums, such as this one, grew in popularity from the 1690s onwards. The practical nature of this work was emphasised in advertisements for it, which described it as ‘very exact, and convenient, not much bigger than a spectacle case’. Its success is evident in the eight further editions that were published up until 1753. It was also heavily pirated, and the numerous errors in counterfeit copies led later editors to sign the genuine books in an attempt to authenticate them.