The ABC of Relativity is a 1925 book by Bertrand Russell that serves as an accessible introduction to Albert Einstein's theories of relativity for a general audience. The book explains complex concepts like the special and general theories of relativity, the interconnectedness of space and time, and the relationship between mass and energy in a clear and understandable way. It was a significant work for popularizing science and remains a relevant guide to the fundamental principles of relativity.