Reddit mentions of Flat and Curved Space-Times

i recommend the following books, all of which give excellent treatment to spacetime and spacetime diagrams:

• general relativity from a to b by robert geroch
  
  
• flat and curved space-times by george ellis and ruth williams
  
  
• space and time in special relativity by david mermin
  
  
• spacetime physics by john wheeler and edwin taylor

I think most have provided the standard intro to GR textbooks at this point, so I'll make a vote here for Flat and Curved Spacetimes by Ellis and Williams. The illustrations, examples, and exercises in there are quite enlightening for one encountering SR and GR for the first time. Also, the annotated bibliography at the end of each chapter is an excellent resource to guide one to more advanced reading.