Google Books now has an amazing collection of early National Geographic volumes spanning the first several decades of the Society’s history (NGS was founded in 1888). The important part: all of them are free and can be downloaded in either pdf or epub formats.

The first ones – Volume 1, Volume 2, Volume 3, Volume 4, and Volume 5.

Many more volumes here. I’m partial to Volume 22, because it has a short section on dinosaurs.

Throughout its history, the Society has also published additional books, maps, and other resources to help advance the educational aspect of its mission and many of those early materials are available online as well with some quick searching: this collection of pictures and a directory of national and maritime flags (as of 1917) are two examples.