And that's just talking about the general storyline. When we look specifically at the mystery plots, well, things get really hazy at times. Sure, the truly fantastic stories will stick around, but even then most readers will probably not remember all the characters that appeared in that one great story ten years ago, or how the murderer pulled off that brilliant locked room murder. You might remember the trick hinged on the presence of an open window in the room for example, or that the murderer used a string, but do you really remember all the details of the mystery plot and how the detective figured out who the murderer was? And that's me talking about the memorable stories: some minor stories I can hardly remember even after re-reading them.
Sometimes though, you have need to check on plot-related stuff. Someone blogging on mystery fiction for example might want to check what that one trick was in that story, or how a certain piece of misdirection was done, or the exact order of events up to the murder. With details like this, a quick read on a Wikipedia entry (if available) is often not enough, so then there's no other choice but to actually re-read the book or watch the film or whatever, and that's assuming you still have access to that particular work (you might've borrowed it from the library for example, or seen it once on TV).
Enter reference books and guides! Books on books (film/TV series/manga/etc) are not rare of course, and there are quite some works in the world of mystery fiction that are specifically about other works and series. Books on locked room murder mysteries are not particularly rare for example, and some even include short summaries about the situation and the tricks used. But even then, it's rather brief, and the focus lies on the "locked room", rather than the story in general. Strangely enough, there seem to be few guides and books on series that focus on the whole mystery mystery plot, as opposed to the more limited technical and more abstract parts (locked room, alibi trick, etc.). Is there for example a book on all the stories of Ellery Queen that give a brief overview of the plot, of the characters (character relation charts!), the mystery plot (how the murderer committed the crime), the clues and how Ellery uses them and things like that?
Case entry from Conan Drill
I have quite a few of these books for Japanese productions, actually. I mentioned how long Detective Conan was, but the publisher has also making some extra money by publishing story guidebooks once in a while, summarizing both storyline and character development plots, as well as mystery plots, detailing the murders in each story and what kind of tricks were utilized by the murderer.
I have a similar book for Tantei Gakuen Q, summarizing the events in the manga and it's really handy if you only want to know how a certain trick was done or how the story tied in to the main storyline. I even have a handy guide for the TV drama TRICK: it was released in 2010 to coincide with the third film and TV special released back then, and it contains handy character relation charts, short pieces to highlight the key events of each story, the various (magic) tricks and illusions used by both the murderers and the protagonist and much more.
Timeline from 15th Anniversary Gyakuten Saiban Series Encyclopedia 2001-2016
The one I'm most impressed by with however is the recently released 15th Anniversary Gyakuten Saiban Series Encyclopedia 2001-2016, a rather hefty guidebook for the Gyakuten/Ace Attorney game series. The book not only has handy summaries and character relation charts for each and every case, it also lists every piece of evidence featured in a story, as well as other key events. But the big one is the case timeline: the events of each case have been plotted on their own seperate timeline, which shows exactly what all the important actors in a story did both before and after the crime. So you can check exactly on a timeline all the things the murderer did before and after the crime, and check where everybody else was at the same moment, but also when the protagonist found a certain clue and how they deduced who the murderer was. The amount of effort necessary to compile over forty different timelines (as there are over forty cases) must have been immense, but it is really appreciated.
I am not sure why these kind of reference books seem much popular with series originating from other media (like TV/games) compared to novel series. I'd love to have a Poirot guide with timelines for each story and character lists and stuff! Who wouldn't want a book on the Ellery Queen novels that feature a summary of Ellery's deductive chains and the evidence which form the foundation for that!? A book that details all the things the murderers do before Lieutenant Columbo arrives on the scene? Anyway, I want my guidebooks!