Illustrators styles and eras.

I just wanted you to see how a single book or series of books could be reimagined by different illustrators over the course of many years and style and decades. What does each cover say about the time it was created and the art styles that were prevalent at the time and what was considered acceptable and sellable to the readers? I'm using Tolkien because these books have been reprinted a bunch of times, you could even find more versions than these. These were the ones I found at a recent books sale. What I want you to think about is that there is no single look for a book or title and the styles you see prevalent right now are passing. There will be new styles and ways of seeing the stories that are written even ones that seem to be so ingrained in stone.