as you too are a collector of sheet music, there may be more progressive works already embedded in your collection than you realize, e.g. inside Henle editions of Scarlatti, Haydn, Mozart Sonates & Klavierstucke, Beethoven bagatelles, Schubert dances etc. One solution can thus be to just start opening books and start skimming for pages with more white page than black ink (or lots of black ink but a largo indication), plomp them down on your music stand and then follow the same routine each time:
1. Quick Scan
- what is the style, likely mood?
- what is the time signature, approp. tempo, feel?
- what key are we in? what does that mean?
- which bits look the most challenging and why?
- how does it start & how does it end?
- what parts are repeated, same or similar?
- what abstract structures can I identify that go beyond "lots of individual notes", e.g. a ha! there is an e minor scale, there is a G major chord, that is a chromatic run, that is a Bflat major broken chord, etc. Those you don't have to read them as notes anymore.
- listen to the music in your head as you scan (or attempt to hum or sing it if you can't)
2. Test Run
- play the start slowly
- play the end slowly
- try out HS the difficult bits
3. Go for it
- set an appropriately slow tempo & stick to it
- start playing (focusing on getting the idea across above hitting all the notes)
- what worked well, what didn't?
- was I ready for this (was it PROGRESSIVE for me?), why or why not, where yes, where no
Repeat for the same piece again, with learnings
Repeat for the same piece again next week
Spending 15-30 minutes of this kind of sight reading practice EVERYDAY (especially never skipping step 1, even if you never go beyond step 1 at times) will bring you dividends to no end (and expose you to lots of interesting music fragments)
The ABRSM sight reading books are often horrible examples of music but do have the advantage of being good indications of what to expect on an exam.
Some useful additions to your collection beyond the core literature mentioned above might be:
- early method books from Alfred, J Thompson, etc.
- the past exam books (& of course the alternative lists from the syllabus which are often filled with great music) from ABRSM at grades that are one or two under your current level
- ABRSM Baroque keyboard collections 1, 2, 3
- Kuhlau sonatines
- Oscar Peterson Jazz exercises, minuets & etudes
- Norton MicroJazz collection
If quick & proper reading of rhythms is a challenge, you could look into doing some remedial work with:
- Sight-read any rhythm instantly by Mark Pillips
- The Rhythm Bible, Dan Fox
Finally, this repertoire grading spreadsheet can be used as an index into your existing collection or of works that you might download from IMSLP, etc.: http://mshartman.iweb.bsu.edu/downloads.htm