Sir Arthur Stanley Eddington 
The Mathematical Theory of Relativity [EPUB ebook] 

Sir Arthur Eddington here formulates mathematically his conception of the world of physics derived from the theory of relativity. The argument is developed in a form which throws light on the origin and significance of the great laws of physics; its consequences are followed to the full extent in the consideration of gravitation, relativity, mechanics, space-time, electromagnetic phenomena and world geometry.
On the other hand, Eddington does a real effort at explaining the basic concepts and their interconnections as he theory unfolds, i.e. the WHAT, the WHY, the HOW and the WHAT IF… And that is so rare that it must be mentioned.