This is a list of articles and specifications about packet radio.
Paper on APRS discusses low level packet TNC operation and specific modifications made to TNC designs to support modern APRS.
While it is primarily focused on APRS, the concepts and issues it discusses are directly applicable to packet.
W2FS's 1998 article discusses using a cheap CPU to send packets.
W2FS’s 2000 article on receiving packets with a cheap CPU
W2FS's 2003 article about the TNC-X (which is modified to be the TNC-PI a few years later)
1998 version of the AX.25 specification is here:
Article on the G3RUH modem for AMSAT
Article on KD2BD modem for AMSAT
WD6EHR Article on radio interfacing for 9600 baud packet