Bartok's Mikrokosmos
Wikipedia has a good summary of Mikrokosmos at https://en.wikipedia.org/wiki/Mikrokosmos_(Bart%C3%B3k)
Essentially these are the initial pieces in Volume 1 (Beginner Pieces) of Bartok's Mikrokosmos for piano
Each piece consists of a Lilypond .ly file, the score in PDF format and a MIDI file for listening
I discovered Mikrokosmos as a very early piano player (pre-Grade 3 exam). Don't think that it alone can turn you into a late beginner or intermediate player. A good teacher plus a lot of dedication can take you from nothing to about a Grade 3 level in just 12 months. And they'll provide much more interesting music than Bartok's early basic pieces. Many teachers use Burgmuller's 25 Easy and Progressive Studies which are more interesting.
Mikrokosmos however is good material for working on sight-reading. You can start at the simplest level and although you'll need quite a lot of other works, you can keep a range of pieces in a collection for a 20-25 minute sightreading exercise. The Bartok number can give you a sense of where you are as you progress.
John Mortensen has a good overview on improving sightreading - https://www.youtube.com/watch?v=GVMW7zo4w90&t=2s
Copyright in New Zealand expires 50 years after the artist's death, so these are all legally transcribed from an old Russian print by me to Lilypond. Check that the copyright has expired in your country.