The dominant aim throughout the book has been to make the ideas definite and intelligible to the ordinary mind. All other considerations strict philosophical phraseology, literary graces, conventional forms of presentation; everything.
"Einstein assumed nature to be such that the total four-dimensional interval between any two events, when computed from event to event along the actual succession, has a maximum value."
No matter in space and time, only intervals between events.
This book with some statements like the above provides a good distillation of relativity.
Argues that the ether is not ruled out by relativity only that it is not needed for it, I would agree if it is qualified that a mechanical ether is indeed ruled out but not some sort of luminiferous ether.