(This is the UK cover, which is way better than the US one.)
There are, depending how you look at it, 2 or 3 versions of qntm's There Is No Antimemetics Division. The original on the SCP wiki, free forever; the self-published book that's a collector's item by now, and the new one published by Penguin Random House, having been through a full rewrite with professional editors, refining it into something much better than its source material. Is it perfect? No. Is the new version worth reading even if you've read the old one? Yes, absolutely.
Below you'll find two reviews: one that's mostly spoiler-free and intended for people who are on the fence whether they want to read the new version or not; and a section full of spoilers, for those who read the new one or just don't care about them.
The one with more details but still mostly spoiler-free
So what changed? So glad you asked. A lot! And it's mostly not just surface-level.
First, for likely legal reasons, they filed off all serial numbers. SCPs are now Unknowns, The Foundation is now The Organization, Marion is now Marie and most other characters also got different names. I don't like this; I feel like Marion was a much better name than Marie for example but it's a small price to pay for all the other, welcome changes.
Said changes include chapters being polished, reordered, added and a substantial part of the second half being significantly different and, in my opinion, better. These make the text and the story flow much better and they address most of my critiques of the book (see the spoiler-heavy section below).
And enough has changed that the new version sits in an uncanny valley where I can't quite say it's a new book, but it is a much, much better version of the original. Reading it feels trippy at times — which is, of course, entirely on-brand for this book — because like, was this in the original? Is my mind playing tricks on me? Wait, why is Marion Marie now?
I'm glad this version happened; I hope TINAD gains a wider audience thanks to having been published through more traditional means and it didn't lose its essence through the editing process. Go read it.
And if you've done so, or don't care for spoilers, the next section is for you.
The one with spoilers
My biggest gripe with the original is that the second half kind of falls off especially with the introduction of "heaven", which for me feels cheap and weakens the story significantly. For me this story element never really matched the vibe of the rest of it. And good news: Heaven is gone.
(what qntm added about the change on twitter)
Sunshine (U-4987) initially lives in Marie and requires a steady stream of memories to be fed. In the new version, it becomes a much more prominent character after her death and saves Adam from U-3125's throes, guides him through the post-apocalyptic "desert" to Marie's dead body in the bunker and then closer to the other Site where Hix (Hughes) has the machine to win the cosmic war.
While Sunshine's rise from a small story element to something much larger could have been perhaps even more polished, overall I like the trope, and more importantly it fits the vibe much more than heaven in the first version.
The reordering of chapters also makes a lot of sense. Chapters like Immemorial, in which Marie takes a trainee up to the big tombstone were moved before "the first ending", while she's still alive. The first ending now happens halfway through, it's built-up better and flows better into the second half that's no longer a sequel, but a natural continuation of the story.
Conclusion
qntm and his editors/publishers took something good and turned it into something much, much better, and that's a pretty big feat. Go re-read it, it's more than worth your time.