Six not-so-easy pieces1997
Einstein's relativity, symmetry, and space-time
by Richard Feynman
These are six lectures taken from the 3 volume Feynmann Lectures on Physics:
Vectors (Vol I chapter 11) Symmetry in Physical Laws (Vol I chapter 52) The Special Theory of Relativity (Vol I chapter 15) Relativistic Energy and Momentum (Vol I chapter 16) Space-Time (Vol I chapter 17) Curved Space (Vol II chapter 42)
— from OpenLibrary
10 editions at OpenLibrary