Welcome to the wonderful collecting field of air mail covers! The history behind these airlines and military aircraft that flew these is colorful and historically important. You can focus very narrowly from a handful of covers or go wider and the number of covers can become almost limitless or be anywhere in between. If your initial interest is Lindbergh, Foreign Air Mail Contract routes (FAMS) and Contract Air Mail Routes (CAMs) the best reference books for you will be the American Air Mail Catalogs. Also, you really will want to joining the American Air Mail Society where you can interact with its members who include "the" experts who write and update the catalogs along with all of the others who enjoy collecting first flight covers of all kinds. And as one additional benefit of being a member you get their monthly Air Post Journal which is a small magazine that focuses on stories about airmail covers. And you also get member discounts on buying the catalogs, or you can pick up some used ones on eBay
. These catalogs are prepared and edited by members who are the experts on given areas of collecting and it can take a number of years for a new edition to be produced with one new Volume coming out every few years. We are currently in the middle of creating the Seventh Edition and so far the first three Volumes have been produced. Not all collecting areas are included in every Edition so for some things you may need to reach back to the Sixth or even Fifth Editions to find a Volume with the area in which you are interested. For example, something like war time propaganda leaflets from WW 1, WW 2, Korea, Vietnam and other wars that were dropped by air are in the Fifth Edition but not in any of the newer editions yet. The same with Zeppelin flight covers. Currently, the newest update on FAM covers is in the Sixth Edition, Volume 3 (which also includes Canal Zone first flights and Alaska first flights). There is a Lindbergh specific section but the newest one of that section is in the Fifth Edition, Volume 3. CAMs are being reorganized into those that are pre-1934 and those that are post 1934. The reason for the break in that year is that is when the US government became so upset with the contracts that they decided to cancel all of the CAM contracts and have the US Army step in an fly the mail. The Army was not prepared and so after only a couple of months of Army flights the Post Office reorganized the airmail routes and re-competed the contracts to fly them. So because there was significant difference between the pre and post 1934 dust-up the Catalogs are finally being reorganized to reflect the differences and the pre-1934 routes are keeping their designation as CAM while the post 1934 versions are being called AM routes. These new and improved Catalog listing for what had originally been called CAMs are found as follows: The pre-1934 CAMs are in Volume 1 of the Seventh Edition. The first half of the post -1934 AMs are in the Seventh Edition, Volume 3. When the new Volume 4 of the Seventh Edition is issued it will include the second half of the AM listings. And if you are interested in those short-term US Army flown mail routes that temporarily filled in between the two, those are all listed in the Fifth Edition, Volume 1 under the heading Army Emergency Flights.
Here is the website where you can not only join the American Air Mail Society but also access super information such as their archive of Air Post Journals going back to 1929 and other great information: https://www.americanairmailsociety.org/