Associativity of Addition is one of the easy induction proofs.
A short example is here.
The following skips associative but does discuss commutative and proof by induction.
More difficult to read perhaps:
Somewhat easier to read, but still fast paced is
A fun document is the following
Each of these has its benefits.
I have set up a short easy to understand book on just associativity of addition. At e-vendor.
This book follows the same proof as the others for associativity. It is however, somewhat more structured and easier to grasp what is used and what is going on.
An interesting blog article