Pet peeve: series authors who introduce things in non-series books!
I was recently reading a series (not a very good one, I don't plan to continue it) where in book 4, the main character is single and at the start of book 5 she has a boyfriend with whom she has been in a relationship for three months who she met on a case. I had read all the previous books in the series and did not remember this at all. I did some research and found out that the author had written a short story which had appeared in an anthology with several other authors. It was in this short story that the case, and the boyfriend were introduced.
Also in the Sookie Stackhouse series, there is one key plot event in one of the books which is based on a short story the author published elsewhere. I went from one book in the series to the next and kept feeling like I had missed something, and again had to do some digging to find out what was going on.
I think this is really not fair to readers. A person who is reading the books of a series in order should expect to be able to follow all the threads in that series. If the author wants to do a side project using these characters, it should not be written so that it has such a material effect on the series that a loyal reader who is going from Book 3 to Book 4 will feel like they have been cheated. As an example, Nora Roberts often writes little side stories using the Eve Dallas character from her J.D. Robb series, but these are just yet another case that detective character works. They don't introduce pivotal new characters or radical changes into the lives of the characters we already have.
Has anyone else encountered this problem before? Are there authors who are particularly egregious offenders who I should avoid in the future? Honestly, even for authors for whom I am a fan, I can't possibly be expected to keep track of every side project they might do. I buy the books in the series, in the order they come out, and feel like given that, I should be able to follow all events and developments with ease. If there is anybody else out there who is doing this, I would rather know ahead of time so I can avoid those books.