In a method body ‘this’ (‘self’) refers to the receiver, the object that received the message.
‘super’ refers to the same object.
The difference is where message lookup begins.
This is one of the few subtle parts of Smalltalk.
There are lot’s of books on Smalltalk, there’s also this perspicuous remark by Eliot Miranda:
https://www.quora.com/What-does-self-mean-in-Smalltalk-exactly-and-how-can-it-be-used