Boogiepop Phantom (2000)
I remember trying to watch Boogiepop with my younger sister because we heard it was similar to xxxHolic in regards to the psychological episodic format, but the style was too confusing for my then 10 year old sister lmao. So we gave up and didn't bother, especially because I later read you need to be familiar with the novels, or the live action, or god knows what before watching it. Well, I watched it now anyway!
Knowing the backstory definitely helps when it comes to piecing together the main, overarching story. I read a bit of the summary, and thankfully in the episode threads on MAL there was a user doing a little info-dump after the first couple of episodes to explain what's going on (thank you SuperLlama). That aside, I feel like Boogiepop Phantom is pretty self-contained. The main "villain" of the show is dealt with in the second to last episode and the story of the show itself ends with that. The last episode is more of an epilogue. At first there's a lot of nudges to the novel and previous events, but eventually they explain them in decent exposition episodes.

The shows main strength is in the way it presents its story and does the character studies, with many characters being interconnected, different perspectives, frequent jumps to the past (which really fits the state of the city where the past and present are mushed together), non-linear storytelling etc. The sound effects and music are amazing, the spooky atmosphere is top-notch, probably one of the best I've seen in anime. Once the whiplash of the bright, vibrant and cheery last episode hits, you can see just how much of a good job was done on everything in terms of presentation in the previous episodes.
Overall, this is a really good anime, especially in terms of execution. The main "flaw" is that if you haven't read the ln, some of the things that move the plot forwards are unclear and in fact the main catalyst for the things happening isn't explained so it feels like you tuned into a movie that's 30% in but you enjoy the ride anyway. That is of course, not necessarily a flaw if you have read the ln, which I probably will now lmao. So the people that said this anime was like a gift to the fans of the source material were right. Nonetheless, it was good enough for me to binge it, something I haven't done in a while.