jonas was a master, his pieces are pretty incredible and i'm always hunting for his scores to scan....thing in modern methods and teaching have watered down the study to such an incredible degree that it slows progress, 'intermediate' seems advanced becuase the learning curve has been extended too far , it will just take time and effort at first to 'catch up' but you have some modern option where the buy in is not quite as high
This method is organized into 18 weeks (or chapters). Each week is devoted to different aspects or problems of piano playing -- patterns, chords, double notes, repeated notes, octaves and more. Each week is accompanied by a Practice Calendar to organize your practice time.

Paperback: 252 pages
Publisher: Ekay/Shacor (March 1, 1996)
Language: English
ISBN-10: 0943748445
ISBN-13: 978-0943748443
about author
(I have this book it's great but be warned 'intermediate' is relative, in many ways it's probably batter to say very late or talented intermediate to early advanced. also , don't hold the 20 minutes at face value, at first and for a good number of them, they will take a bit longer to work through that week's daily exercises/workout
https://www.naxos.com/person/Douglas_Riva/638.htmDOUGLAS RIVA
The American pianist Douglas Riva has gained international recognition for his profound knowledge of Spanish music, and no less an authority than the distinguished Spanish composer Xavier Montsalvatge has described him as an exceptional pianist. Douglas Riva’s interpretations of the works of Enrique Granados, in particular, have earned him a place as one of this composer’s leading exponents worldwide, praised in the Spanish press and elsewhere. His recordings for Naxos of the complete piano works of Granados have won critical acclaim, notably in Spain, the United Kingdom, Japan and the United States. Douglas Riva is the Assistant Director of the eighteen-volume critical edition of the Complete Works for Piano of Enrique Granados, directed by Alicia de Larrocha and published by Editorial Boileau, Barcelona. An active recitalist, he has performed at the White House and at Carnegie Hall and has recorded numerous programmes for television and radio in Brazil, Spain, Portugal, Holland, and the United States. He gave the first American performance of a newly discovered Scarlatti sonata at the National Gallery of Art and the world première of Gazebo Dances by John Corigliano in Barcelona. Granados’s long-lost masterpiece Cant de les estrelles for piano solo, organ and choruses was performed for only the second time in history by Douglas Riva and the Voices of Ascension, directed by Dennis Keene in 2007. The Naxos recording of the première performance, Song of the Stars (Naxos 8.570533), was nominated for a Grammy award. Douglas Riva started his musical education at the age of nine, studying both the piano and the flute. He began his professional career at the age of sixteen as the Principal Flautist of the El Paso (Texas) Symphony Orchestra. Later, devoting himself exclusively to the piano, he continued his studies at the Juilliard School, New York University, and at the Academia Marshall, founded by Granados. The Fallen Nightingale, a novel by John W. Milton based on the life of Enrique Granados, was published by Swan Books in 2005 and includes a compilation CD of Douglas Riva’s recordings for Naxos.
PS
chopin etudes- no
Bach - yes
Beethoven Sonatas - yes