Wim's little book is a gem. I also got Alistair Anderson's. That one was nice because it came with a CD. There are several places you can get it, but Elderly Instruments is the only place I've seen the book and the CD both:
Book; Recording
Not that there aren't other places. Just haven't seen 'em.
Sounds like Mr. Wakker ought to start selling his concertina tutorial for $20 a shot. =)