Mark Dittmar's site: Hartley's, Regens, and an MOPA.
An excellent few pages describing an early style TNT (Tuned-Not-Tuned) transmitter:
Electric Radio Magazine's site.
Ben Tongue's Site.
Ben was the co-founder of the Blonder-Tongue Company, and is a very knowledgeable, and fine gentleman.
Technical talk about Crystal Radio Design, Late 1920's TRF Radios, Antique Hearing Aids and Blonder-Tongue Labs