OK, having looked.into it casually for a few.minites, limited.to just stuff from 1900-1923:
- A ton by Lord Dunsany: King of Elfland's Daufhter was 1924, but he did a lot.of fantasy short fiction prior to thar.
- A Voyage to Arcturus by David Lindsay
- A Princess of Mars, by Edgar Rice Burroughs (among other works)
- The Lost World, by Arthur Conan Doyle (among other works)
- Fantômas, by Pierre Souvestre and Marcel Allain (pretty fantastic, extremely influential on the superhero genre)
- The Worm Ouroboros by E. R. Eddison
- The Wonderful Wizard of Oz by L. Frank Baum (among other works)
- Peter Pan; or, the Boy Who Wouldn't Grow Up, by J. M. Barrie
Definitely stuff still worth reading, still actively influencing modern fiction.
A LOT of Nickel and Dime novels and pulp books (one of the most famous in the late 21th century but interestingly enough has seemingly been forgotten in this decade was Tom Swift for example). It was extremely abundant, though what they considered Fiction with a scientific idea of what the future could hold back then might not seem so miraculous to us today in some ways.
It was probably more abundant or just as abundant today, but many just don't remember the stuff unless they specialize in research of it either professionally or as a hobbyist.
Even stuff from the 30s, 40s, and 50s are hard to find today (but some of the biggest influences on the late 20th century scene came from that period).
How many of them are MUST reads though...it's not enough to just be remembered...there must be a reason for it to be a MUST read as well.
What is interesting is back then they had societies of authors which specialized in this type of stuff. Many times what they wrote was specifically for society members. Normally the stuff we have were written for the pulps or dime novels or the cheap stuff that got VERY popular.
In effect, the stuff we remember is what was EXTREMELY popular back then and something came of it (for example, books which had movies made of them that were extremely popular sometimes adds to the book remembering item. The Wizard of Oz is a great example of this. Many today would say the Wizard of Oz is a notable book and thus for our arena of Fantasy...a MUST read. Now I read the entire series as a child, but for much of the late 20th century the OZ series could not be found in libraries or bookstores. If one even realized they existed they had to special order them and then might not be able to obtain them. If you were lucky you might find the Wizard of Oz Sequel (The Marvelous Land of OZ) but even then, it wasn't common.
It's had sort of a revival over the past two decades, but even now, most people aren't even aware that there are more books than the Wizard of OZ, and if they are they don't know what most of them are called. Of the OZ books, the only one that MIGHT be considered notable thence, would be the Wizard of OZ. It isn't the series, but that singular book. IMO of course.