I am looking for good references (pdf papers) that discuss and justifies the use of the "modified" dot product: \[ {\langle}A,B{\rangle}=-A^0B^0+A^1B^1+A^2B^2+A^3B^3 \] in the 4-dimensional space time metric (Lorentz, Minkowski) by way of basic principles using just ideal clocks and light signals. I tracked down A.A. Robb (1936) but that was not useful. I am looking for papers written at the advanced undergraduate level or at the beginning graduate level. Any such references would be appreciated. Thanks.