Hiya Brian
Old publications I've seen tended to have any number of Glow-Bug style transmitters.
The ones to look for are the designs that use Cathode Keying i.e. the Morse Key was inserted into the Cathode connection of the Tube (Valve), and for AM use the key was simply unplugged and replaced with a carbon microphone. The RF was generally bypassed from the mic with a small choke and a low value capacitor (around 10nF~100nF).
That system worked fine for MOPA (master oscillator power amplifier) circuits, most MOPAs were little more than a Triode Tube and a crystal to control the frequency, arranged in a Colpitts configuration. I've even built free running oscillators in the past (Hartley Oscillators), and used the same idea of inserting a carbon microphone into the Cathode lead.
Another way to amplitude modulate a simple oscillator is to vary the Anode Voltage. In the past it was possible to obtain purpose built modulation transformers, but these days they are a little hard to come by. Having said that it is possible to use a standard mains transformer as a substitute. All you have to do is insert the transformer primary (the winding that would normally go to the mains 110/240-Volt input) into the Anode circuit so that the high tension DC passes through the primary winding. Connect an audio amplifier to the secondary winding of the transformer, and as the audio voltage from the amplifier is stepped up by the transformer so the Anode voltage will vary and there you have it, Amplitude Modulation.
There were other methods too, like Choke Modulation, Screen Modulation etc etc.
The very first AM transmitters were high powered oscillators with modulation being achieved by inserting either Carbon or Point Contact microphones in series with the antenna lead so as to vary the attenuation (or coupling if you like) between the TX circuit output and the antenna.
Best 73, Mark.
