Reddit mentions of Super Sight-Reading Secrets: An Innovative, Step-By-Step Program for Musical Keyboard Players of All Levels

Idiot proof. It will take time, though.

https://www.amazon.com/Super-Sight-Reading-Secrets-Step-Step/dp/0961596309

There's nothing wrong with your motor skills. If you can wait I'll be addressing exactly this problem in my sight-reading series but for now a short version of the answer: the problem is that, even when you're playing without looking at your hands, you're still going through the works in your brain as if you are looking at your hands. That's why, despite practice, you're only seeing marginal improvements at best.

Playing without looking is not just doing the same thing blindly, there's a whole other mechanism that triggers the movements. This doesn't matter when you're only sight-reading simple melodies but it becomes an increasing hurdle as the music gets more complex. That's why so many pianists wrongly conclude they just lack the "natural" skill to sight-read. They got this far and, even with practice, can't seem to go further.

For now, I suggest you check out this book to get you started. Few things to note though: 1) It's akwardly structured and can get frustrating to work with and 2) I don't think he really understands the problem I described above and would personally suggest taking it a lot further than he does and practice his exercises in a different way.
But it's, for now, the best place to start. Oh and be patient. He says himself it takes anywhere between a few months and a few years. I would give it at least 6 months.

I would second getting a teacher, because even if you manage to learn how to read/sightread music well, you will 100% have mistakes in your technique that will become very difficult to correct later and can cause long-term damage to your hands if you don't.

A good option is to get a teacher for ~3 months to teach you the basics and ensure proper technique. After that you can stop the lessons and move onto following this guide. This is one of the best guides I've seen (shoutout to u/clarinetist001) and really helped improve my sightreading.

I would also recommend checking out [this book] (https://www.amazon.com/Super-Sight-Reading-Secrets-Step-Step/dp/0961596309) which you can read online, if only to get an idea of the mindset you want to be in when sightreading, as well as some basic drills to get you familiar with the keyboard.

TLDR: Teacher for 3 months, follow this guide, enjoy being able to sightread well.

I spent many years without being able to sight read at even 20% the speed required on most of the pieces I learnt.

But for what it's worth, I recently spent a month or two on the book "Super Sight-Reading secrets" (don't bother the catchy title) and I made tremendous progress in sight reading. After some really basic lessons to recognize the notes, the drills are separated in "keyboard orientation" drills and "visual perception" drills. Those former ones really made me improve ; and what is really weird is that the most progress I made was only by working several hours on one single sheet full of smartly positionned notes on 4 lines of piano staff. My level was really bad though, and I guess at higher level this book does not help much.

Bach chorales for piano are also advised in the book.

My teacher told me to try out Czerny's pieces too.

edit: add link