Reddit mentions of Pianists Guide to Standard Teaching and Performance Literature

This is the best book I have ever used. It grades so many pieces (you will never finish playing) and has a few sentences explaining the piece or what it's about. Extremely useful index by historical period and composer, and find what pieces are accessible to you.

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There are multiple sources:

• Snell's Repertoire Index
  
  
• RCM Syllabus
  
  
• Jane Magrath's guide to standard repertoire
  
  You can also pick from any of the carefully graded repertoire books - similar to the Kjos series above. Some of those even have accompanying study guides. Here's a nice example.

If you're looking for books that are good collections of pieces, I'd recommend the [celebration series] (http://www.sheetmusicplus.com/title/celebration-series-perspectives-piano-repertoire-1-sheet-music/18355179) for collections of classical works. There are different volumes for different levels, I just provided a link to their "level 1," although there is a primer level if you want even easier. If you want additional repertoire resources, you can also find a lot of older (public domain) music on IMSLP (international music score library project IIRC).

For more repertoire help see if you can find a copy of Jane Magrath's Pianist's Guide to Standard Teaching and Performance Literature. It'll give you ideas on repertoire for years to come.

Lots of research and personal experience.

Up until last month, I was constantly running out of things to play because I played a high quantity of pieces, but I was limited in what I could play due to me not being a very good piano player. Except for the Swinstead and Gurlitt Op. 140 and 205, I've played at least some pieces from every book on the list, thus I'm able to accurately judge the difficulty of these pieces.

I've found a lot of material by reading through Jane Magrath's book on repertoire for beginners, my teacher provided me with some suitable material, and I recently bought pretty much every book from ABRSM's collection of 'Easier Piano Music'.