Sorry sent last post in error. Once again Melodic Adventures In Bass-Land for the Accordion, By John Caruso. By Alfred Publishing Company. This publication pretty well covers the bass side of the piano accordion. First row from bellows- counter Bass-2nd row fundamental bass - third row Major Chords - Fourth row Minor Chords - Fifth Row DSominant 7th Chords - sixth Row Diminished 7th Chords.
Fingering usually starts with ring finger on the fundamental bass, the little finger is used for counter bass, the middle finger for major chord and the first finger handles minor 7th and Diminished.
The basic idea is to play a bass accompiment roughly in the same manner as playing guitar accompiment. Fundamental bass and then the chord and bass runs and alternate bass lines built with the counter Bass. Really requires extensive study and would take too long to explain all here.   Hope this helps a little.