BartK The Four Star series is absolutely amazing. I thought RCM discontinued this series and only has the online app, which is embarassingly bad, but maybe they are still available. These books show you how to sight read with short exercises in rhythm and note reading. Each book is appropriate to the sight reading skills expected at that level, so for example the exercises in level 3 are much easier than repertoire of level 3. The preparatory books start with reading one note at a time and finding all the As, Bs, etc., like you would as a total beginner, and the level 10 asks you to basically sight read a page from a Beethoven sonata or a Debussy prelude or similar stuff (that's just so you have an idea of the range of these books). I bought these books 8 years ago and am still using them to this day (I'm on level 10 if anyone is curious
). If you can get those books then definitely do so.
The full set I ordered from eBay just landed on my front porch! Now I need to figure out how to slot sight reading into my daily practice routine.