What I wish I knew before I dove into the concertina pond:

[Cornfield]

I just found the ABC program https://michaeleskin.com/abctools/userguide.html

what a great resource! I can read music already so this is a godsend. I would rather read music than use tabs eight days a week.

[Cornfield]

6) Playing major and minor thirds on the English concertina is soooo easy and sound great.

[Duncan Luddite]

G'day Cornfield, I'd recommend:

The Concertina: A Handbook and Tutor for Beginners on the English Concertina

by Frank E. Butler

It's a free PDF download from this page (a little way down in the right margin), then print it out. It's been invaluable for me:  http://www.concertina.com/merris/bibliography/english-tutors.htm
Cheers