If you want to understand interest rates, a sweeping panorama is presented in Sidney Homer's A History of Interest Rates, 2000 B.C. to the Present. (There's a fourth edition out, available on Amazon, but I like my signed first edition, published in 1963.)
Chapter 1 starts out, "In historic times credit preceded the coining of money by over two thousand years." Think about that; loans at interest before there was even money. (If that's not obvious to you, answer at end of post.)
Here's an interesting discussion of cash-out mortgages in California during the Gold Rush, and the consequences for the property owners:
OK, if it's not obvious how you have credit without money, imagine one farmer lending another farmer a bushel of grain, to be repaid after harvest with a bushel and a half.