It's not specifically a text on software design, but Sunburst and Luminary by Don Eyles is an enjoyable read ( https://www.amazon.com/Sunburst-Luminary-Apollo-Don-Eyles/dp... ). He manages to capture the Apollo-era zeitgeist in more ways than one. Not just another tale from the trenches.

Sunburst and Luminary [1] mentioned using the native LGC assembly for some performance-sensitive parts of the flight; do you know how common that was/how much of the code was written that way?

[1] https://www.amazon.com/Sunburst-Luminary-Apollo-Don-Eyles/dp...

Eyles has written a book Sunburst and Luminary: An Apollo Memoir, https://www.amazon.com/Sunburst-Luminary-Apollo-Don-Eyles/dp..., published earlier this year which recounts his role in designing and building much of the software in the Apollo guidance computer. It recounts much of this history and is a worthwhile read for space and Apollo junkies.