Just gone out and bought her second book as a belated birthday present to myself.
(For anyone new to the thread, I'm in Germany and the book is in German. An English translation is not out yet.)
Just started it. The intro says it is definitely meant to be a follow-up to the first book. It covers various problems that people have clearly been facing along the way, as well as maintaining a decluttered home and strategies on cherishing items that are loved but not necessarily useful.
Various issues she mentioned are ones that that have come up here: like how to figure if something sparks joy, how to deal with what it is needed but not joyful. What to do if you start cluttering again despite having decluttered already. It also has more details on folding (with pictures) and storage.
From the little I've read so far, I'll summarise issues she've mentioned that have been asked earlier on here, sometimes several times.
How to figure if something sparks joy: you should have all of one category of item in front of you, ready to go through. Try picking out your three favourite items from the pile. Now rank the three in order of most to least favourite - (she warns this may take a while if you have never done this before). The feeling you get with those items should help kick-start a basic feel for how much joy, or not, you get from other items.
How to deal with what it is needed but not joyful: if it does its job and makes your life easier, and not having it would make your life harder, then it should probably stay. In its way, it is bringing joy. As a side-note to this, she mentions that most people who put their work clothes into this category usually come round to the view that it is actually the work they do which they dislike.
If clutter starts again, she's pretty adamant that you must not have followed her instructions fully in the first place. She really stresses that you should do the sorting in the order she gives, should NOT clean or start putting away items until the sorting into joyful and non joyful has been done, and should NOT do by room but by category.
I'll write more as I keep reading but - apologies in advance - it will be in dribs and drabs as I have work deadlines, a week of school holidays is coming up and, on top of all that, our Internet connection at the moment is also tending to come and go despite a technician having come out fairly recently.