This list is for books which were written and published after at least one other book in the same continuity by the same creator(s), and which voters felt were a big disappointment based on the previous book(s).

Voting guidelines (please read):
-Remember that the criteria are "Most Disappointing", not "Worst". When voting and when ordering your votes, you should prioritize books which were the biggest let-down compared to their predecessors, not the ones with the lowest overall quality. So a mediocre follow-up to your favourite book of all time should be ranked ahead of the exceptionally poor sixth book in a series which was always low-quality but mildly entertaining, for example.
-As noted, the sequel/prequel must be by the same author as, or more exactly share at least one author with (to include things like graphic novels with the same writer but different artists) the original good book(s). This is to keep the size of the list down, as otherwise a very large pool of "published fanfiction" for older out-of-copyright classic books becomes open for inclusion, along with things like cheap cash-ins written by an author's children and sequels by different people in shared universes (Marvel, DC, Star Wars, etc.).
-Again to keep the size of the list down, please only vote for the first book in a series which you felt marked a significant drop in quality (or the point where the series had "jumped the shark"). So for example, if books 1-6 in a 10-book series were great but 7-10 were terrible, only include book 7.
-As the title indicates, this can include both prequels and sequels. Alternate-universe versions, POV sequels and the like can also be included - any disappointingly bad book published as a follow-up to any notably good book(s).