Transmissions,Differentials, Gear Ratios....

There's plenty of info out there about building engines and making power but I can't seem to find much on trannies. How they work, how gear ratios effect performance, whats better for N/A, whats better for FI? No one really ventures past clutch and flywheel tech. I think this is often overlooked and I for one would like to know more. Thank You.