For the purposes of sightreading? I can't think of any off the top of my head for piano. I've definitely found plenty for monophonic instruments. Most of the reading I do that's harmonically dense is in the relevant styles. And since a lot of harmonically dense music is in styles that are improv heavy, it's hard to find explicit sightreading material for them and most of it's just sightcomping from chord charts.
The few places I run into it are some jazzier/bluesier choral octavos and books for various musical theatre shows.
As much as I have mixed opinions about "jazz" arrangements, most of my problem has to do with the fact that people whose goal is to learn jazz tend to not learn much from using them as a starting point... but if you wanted to intentionally read stuff that's denser, you could look into some.
This series has some great arrangement but they are well beyond what most would find comfortable sightreading. They are beyond what I would find comfortable (a mostly consistent half-tempo).
If you want to fall somewhere in the middle there are all sorts of modern song books. Hal Leonard has some. "The Best Broadway Songs Ever" And "The Definitive Broadway Collection" are obviously some theatre tunes so some of them will fall into very standard modern pop harmony while some will actually be pretty dense and jazzy with plenty in between. They also have their decades series books that cover pop songs of those decades, but a lot of times they are pretty standard and the more modern pop stuff doesn't lend itself to piano arrangement. Like... the melody line is just one pitch with a weird rhythm for 4 bars if you've listen to much modern pop music lol.
Various song book collections as the best bang for the buck, especially if you find them at a used book store (which is where I tend to find lots of high volume material for reading).
My other go-to isn't actually available to people widely... The various publishing companies show up at music conventions and try to sell their new choral music. To do so they have "reading sessions" where a room of hundreds of choral directors are given these bound copies of dozens of new choral pieces. The goal is to have all of these trained choral teachers sightsing through the selections (with an accompanist on stage) and they get a pretty good idea of how all of the new stuff sounds and can decide to buy sets of single octavos for their classes if they like some of them. And some will keep these in their libraries to revisit when shopping for new tunes.
But I just love the bound collections and snag them from the reading sessions... and with several publishers all holding several sessions I can amass hundreds of octavos at a convention bound into maybe a dozen books. And I also ask choir directors who have any they don't want to give them to me... or have choir director friends intentionally grab duplicate copies for me to use as reading material.
There's actually a surprising amount of choral music that's pretty dense... the sacred choral music these days is WAY more intense than most of what the secular (i.e. written for school programs) choral music is. That's where you can find the composers letting loose because there are so many programs who have absolute veteran accompanists and huge volunteer choirs so the writing can be pretty sky high. That's where you start finding lots of mixed meter, asymmetrical meter, very contemporary-but-not-necessarily-jazz harmony, ridiculous jumps, runs, bombastic shit left and right.
I just wish it was more easily available, especially for people wanting to work on choral accompaniment because every little music is actually written like that sort of accompaniment music... except that type of accompaniment music.
Another place to check if you want to walk on the grey side is /r/musicalscores3 (after the first 2 got banned for obvious reasons) where people share sets of books for musicals...like the actual books for the entire pit. As shady as it seems, most people aren't snagging these books to throw unlicensed versions of these shows (which is ostensibly why the publishers try to stamp out the piracy). It's mostly just pit musicians who want to get access to books well ahead of a show. Like the theatre company my wife and I do most of our work for announce their season a year ahead and most of the schools announce their shows many months out.
But they have to pay to rent the books for a period, so sometimes they don't even have the actual books until as little as 1-2 weeks out in extreme cases (had one of those this summer... fml). Musicians just want to get their books way ahead of time because most of us working musicians will have LOTs of gigs overlapping and it's fucking hard to prepare 100-300 pages of new music in just a few weeks WHILE prepping for other gigs simultaneously.
I've definitely just read through some scores for fun. NOT light sightreading, but very true-to-life for anyone interested in ever doing that type of work to see what it's actually like... and maybe just good dense sightreading for others.