I've had this book for many years. At first I had trouble getting into it. Once I finally figured out the core idea, I got a lot out of it.Lewis Carroll Epstein develops a wonderful and ingenious technique (Epstein diagrams), but he introduces it by way of a myth (itself quite nice). He never explains clearly how they do NOT depict events in spacetime! Minkowski diagrams DO depict events in spacetime, but they pay for this advantage with a drawback: the *metric* of spacetime is not depicted correctly. In the rest of this review, I'll explain this as clearly as I can in a short space.Because I want to keep the explanation brief, I'll assume some background, and stick to special relativity. I'll assume the following all makes sense to you. (a) Spacetime is the collection of all events. (b) Each observer will assign spacetime coordinates (t,x,y,z) in their own way to events. (c) In two dimensional diagrams, we have to leave out two spatial dimensions, say y and z. (d) Suppose two events, E1 and E2, occur close together in time. Say they are assigned coordinates (t,x) and (t+dt, x+dx) by one observer (so dt and dx are very small). Another observer will assign coordinates (t',x') and (t'+dt', x'+dx') to the same events. Now, as just noted, the regular and the primed coordinates will usually differ, but the following equation will hold:c^2 dt^2 - dx^2 = c^2 dt'^2 - dx'^2, or if we choose units where c=1dt^2 - dx^2 = dt'^2 - dx'^2(This was Minkowski's big contribution.) (e) If both events E1 and E2 occur on the worldline of an observer (say the primed observer), then dx'=0 and dt' is the so-called proper time between the events: the time that passes from E1 to E2 according to someone who is "right there" for both E1 and E2. Proper time is traditionally denoted tau, so dtau is the proper time from E1 to E2. So we have:dt^2 - dx^2 = dtau^2In a Minkowski diagram, each event in spacetime is represented by a point. We draw a vertical t-axis and a horizontal x-axis, and put dots somewhere for E1 and E2. Pitfall: the Euclidean distance between E1 and E2 is the square root of dt^2+dx^2, so Minkowski diagrams do not represent time intervals accurately! If you take a ruler and measure, on the paper, the distance from E1 to E2, what you get doesn't tell you anything useful (in general) about the proper time from E1 to E2.If E1 and E2 are not close together in time, you get the proper time tau between them by integrating dtau along the worldline from E1 to E2.Now Epstein had a clever idea: he rewote dt^2 - dx^2 = dtau^2 as:dt^2 = dtau^2 + dx^2Epstein draws a vertical tau-axis and a horizontal x-axis. Now the paper measurements tell you something useful right away: the coordinate time between E1 and E2. But there's a fly in the ointment: points in an Epstein diagram do NOT correspond one-for-one with events in spacetime.See the picture (of the twin paradox) at the bottom of this review to show the difference. In the Minkowski diagram, we have two travelers who go from event E1 to E2. E1 is where the twins part, and E2 is their reunion. Drawback: although the traveling twin's worldline *looks* longer, it is actually *shorter* (that twin is younger at E2). In the Epstein diagram, there are *two* points for the same reunion event E2. That's because the proper time along the traveling twin's worldline (his tau) is less than the proper time for the stay-at-home twin.There is really only *one* reunion event E2 in spacetime, and so there is only one t coordinate for it. In the Epstein diagram, this means that the *length* of the curved path should be the same as the length of the straight path from E1 to the upper E2.I think of an Epstein diagram as a graphical way to compute coordinate times along a worldline. (Look up "nomography", if you haven't seen this word before.) I think it's remarkable how Epstein has developed this idea to include so many aspects of special and general relativity. If only he had spent a page or two contrasting his diagrams with Minkowski diagrams, and explaining the difference!From a theoretical standpoint, Minkowski diagrams are fundamental, or at least Minkowski's conception of spacetime is the key concept. The duplication of events in Epstein diagrams has no physical significance, sort of like the duplication of cities on the left and right edges of a map of the world.