There are a few that are incomplete due to the fact that their authors passed away before they could complete their work, but I don't know about any that were intentionally abandoned. It's easy to hate on George R. R. Martin and say that he has abandoned his Song of Ice and Fire series, but I think he is really trying to do his best despite a lot of external factors that are against him.

The author is very adamant that he didn't abandon the series, but everyone who started reading The Kingkiller Chronicle by Patrick Rothfuss is getting a little impatient at this point. Granted, it's only been about eight years since the last book which is not as bad as some of the other books mentioned here, but it still a bit galling. To put it into perspective, I started reading the first book, The Name of the Wind when my son was born and he's almost a teenager now. I guess will have to wait for the Sam Raimi film to get some type of conclusion to the story, provided that ever materializes.

I only recently discovered that Mark Z. Danielewski, the guy who wrote House of Leaves appears to not have had that much luck afterwards. House of Leaves has rightfully gained quite a cult following amongst readers, but then he attempted something different with "The Familar." It was supposed to be a 27-volume serial novel, but Danielewski got up to volume 5 before his publisher pulled the plug on the project. Apparently there were not enough readers into these books to justify the continuation of the series. Someone who has actually read any of the books might be able to shed more information on why they were not that good, but personally I don't like starting on things before they are completed. This is also why I haven't read the Song of Ice and Fire series yet, although I did cave in and binge watched the entire television show after it concluded.

Frank Herbert never managed to complete his Dune series, but I believe that his son and another another eventually managed to do so. Whether or not the quality is the same as what Frank would have delivered is up for debate. The Wheel of Time series is another good example of someone else stepping in to complete a beloved series after the untimely death of the original author.

This one is a bit obscure, but the Night World series never received the final book as far as I know. It's from the same author who wrote The Vampire Diaries and she released nine books in the series, but it's been 21 years since then with no sign of book ten. The series itself is actually very good and takes place in a world where there are all kinds of witches, werewolves, vampires and shapeshifters living secretly amongst ordinary humans. I would actually love to be able to complete my collection although I haven't read the books in years and I don't even know if the author is still really interested in finishing the last book.