I have a copy of the Sally Army "Triumph" tutor at home.
As I also have a Crane system duet box I guess I should take a look at it to see if there's anything useful.

Of course, any collection of simple tunes (for any instrument) will be a good start for all types of concertina and just playing the melody is going to get you a long way in finding your way around the instrument. However, if it's a duet system you'll be wanting to add bass lines and harmonies. There are lots of websites with general info but not very much about the Crane system.
www.concertina.net already mentioned
www.concertina.info
plus loads of links from those.

(good luck!)
Anahata