This is a category of literature dear to my own heart and also perhaps to those constrained in reading time. Novellas are a great way to expand the breadth of your reading, so this challenge could prove a useful guide to that end.
According to Goodreads , "a novella is a fictional prose narrative, as a rule of thumb more than 20,000 but fewer than 50,000 words in length. These boundaries are approximate....."
* Some of these books are collections of novellas with the book itself several hundred pages long.
* "Una Historia De Ayer" appears in both position 10 & 150 on the source list, so it is difficult to know its actual popularity with the Spanish-speaking community. It does not appear to have been translated into English.
* I have removed some books grossly exceeding the criteria above; also some duplicates.