Just yesterday I downloaded a copy of Don Lancaster’s The RTL Cookbook from his site, in contemplation of moving away from this modern TTL stuff into real old-school logic: Resistor-Transistor Logic (RTL) and/or Diode Transistor Logic (DTL).
It seems I’ve been beaten to the punch and someone’s taken the logical next step: LCPU - A CPU in LED-Transistor-Logic (LTL).
I recommend starting with the first journal post and working forward. Tim does a good job of explaining how gates work in RTL, DTL and LTL: it’s both interesting and educational.