Richard Graham has written four excellent books on the SR, even though he only says three on the podcast
SR-71 Revealed: The Inside Story
SR-71 Blackbird: Stories, Tales and Legends
Flying the SR-71 Blackbird: In the Cockpit On a Secret Operational Mission
Also, anything, and I do mean
anything by Paul Crickmore on the SR-71. One of the most amazing things about his copious knowledge of the aircraft is that he never was associated with the program!