Airlines of Pan American Since 1927 by Gene Banning (2001)

 

A more comprehensive history of Pan American World Airways will likely never be written. The author was a Pan Am captain, in whose name (along with Dave Abrams) is granted an annual research award by the Pan Am Historical Foundation. An absolutely invaluable reference, and fun reading to boot.

https://www.amazon.com/Airlines-Pan-American-since-1927/dp/1888962178