OK, thanks Amanda and DL/BC for the clarifications. I actually like the librarian policy on "normal" abridged and unabridged staying together, as it seems more intuitive to me. The problem that I have seen is that the derivatives sometimes have the same title and author, in the case of children's books especially, and thus are difficult to distinguish in searches from the real thing. I guess they usually should also have an editor or creator listed to provide a clue that they are different.
As for the Series/Lists topic, when there are exceptions to the normal ways to handle them, it would be great if Shelfari would provide a text box at the top of the page to allow for a short description of the ground rules for a given series, so editors could know right away how to deal with them consistently. In fact, that would be a welcome addition in general, since many lists or series would benefit from a description of how the list came into being and what belongs there, especially the authoritative ones.
Now I also understand what I have seen on some series/lists, where unfortunately, in a numbered list, there are numbers missing, which I at first tried to correct by adding them, and only then noticed that they show up at the end of the list without a number, and also without the header that you normally see. I agree, that is a shame.
I think I will leave RDC Books alone for now, as on further investigation, I noticed that they seem quite messy. There doesn't seem to be any standard naming conventions, or at least they are not consistently followed, and I don't want to introduce more confusion. Looks like the follow-on series "Reader's Digest Select Editions" is partly mixed up with it, but mostly just stand-alone, and apparently not organized into a series of its own.
It does bother me, though, that some of these RD books either have no authors listed or are given only one author, though there should be several normally. One such case is: http://www.shelfari.com/books/12698372/The-Da-Vinci-Code-Up-and-Down-in-the-Dales-The-Return-of-the-Dan - where Dan Brown is listed as the only author, and as a result it shows up on his author page as if he wrote all four. Not sure how to ask for a fix for this, or if there is one.
Again, thanks for the responses and all your hard work to make Shelfari a fun place to keep track of books.