If you’re interested in learning more about leathers – the various kinds, various properties and much more – there’s few better places than The Leather Dictionary. A sort of Wikipedia for all things leather, an excellent source for knowledge.
The Leather Dictionary is built by the German leather care specialists Colourlock (so it’s not a totally unbiased, neutral website, which can be noted to some extent, especially when it comes to info on leather care), and it really is a page with loads of information. Almost 500 articles can be found, on various topics, everything from leather production and types of leathers and its properties, to different leather products and how to care for various kinds of leathers. It’s not at all focusing on shoes, it covers basically all use of leathers. The length of the articles vary, some are short explanations, some are very long with different sections and includes lots of pictures and films.
If you’re interested in going in-depth on the material that all quality shoes are built more or less exclusively of, in various forms, this is a gold mine. I have spent hours clicking around the site, since it’s cross-linked all the time when various topics and words that have their own pages, it’s a bit like being on YouTube where you always find new stuff to head to. Being a German site, it’s function over form, ain’t the pinnacle of design and looks we have here, but it’s logical and easy to browse. And there’s a lot of informative pictures and also films, where I especially appreciate the many macro images of various leathers, along with good explanations of what you are looking at.