The overall best reference is the American Air Mail Catalogue which includes Pan Am flights in several sections. Most of the listings for earliy Pan Am are found in the "Foreign Air Mail (FAM)" listings, and the listings for newer Pan Am first flights are in the "First Flights US Airlines" section. There are also some listed in other sections such as the Canal Zone first flights, the Lindbergh first flights, etc. Pan Am was such a huge airline that all of its first flights spread across many areas.
If you are not already a member of the American Air Mail Society I strongly recommend joining as it is where so many experts on Pan Am first flights reside and they are happy to share their knowledge. As a member you would also get full online access to the Air Post Journal including its archives which is a monthly journal dating back to 1929 and which are a superb source of articles and details of first flights, including Pan Am's of course, and that were written back when the flights were being flown as well as in retrospect. And you would have full online access to the Aero Philatelist Annals which are a monthly journal from 1953 to 1982 and provide additional information to that found in the Air Post Journal on first flights of that era. And you get a member discount on the Catalogues and other books and aerophilatelic references. Check out the Society's website at https://www.americanairmailsociety.org/
and consider joining in the fun and sharing of your common interest.