I logged into KDP a few days ago and found the links to the new series manager.
Today, I noticed there is big link to it on the main Bookshelf page.
I’m glad to see this. I once had a problem with one of my series books not showing up as part of my series, all because of an apostrophe. I didn’t enter the series name differently in the book details, but somehow the system did something totally weird and messed it up anyway.
Maybe now if something similar ever happens again, I can deal with it on my own. :)