I wrote my Master's thesis on cryptocurrencies, and had to do some background research on how the monetary system works. I found that Bernard Lietaer's "Money and Sustainability - The Missing Link" was easily approachable and had a lot of good stuff in it. Another of my staples was David Graeber's "Debt: The First 5,000 Years" which has a lot of good and interesting content, but is very very heavy to read.