Design Principles Behind Smalltalk (1981)
If you're tired of hearing about the virtues of Smalltalk (like I was at one point) and ask "if it is so good, why isn't it popular?", then watch @deech's talk (Aditya Siram) - "What FP can learn from SmallTalk" - https://www.youtube.com/watch?v=baxtyeFVn3wI think it is the most accessible explanation of the marvel of Smalltalk,  for those who were not lucky to work with it during the late 80-90s.Also I think Ruby is the mainstream language that is closest to Smalltalk today, with the idea that everything is an object and late-binding as much as possible.One thing I can't help point out is that Smalltalk / Alan Kay's vision of interconnected objects forming a recursive computing system is not the only "true" vision of OO out there. From Simula thru C++ and then Java and C# also are object-oriented (or class-oriented for those who care about the distinction), but they are statically typed. It is also OO because OO is a word, not a mathematical definition, and when people talk about OO, there is enough similarity between all these variations that we consider all of them to be in the same camp. Plus with the growing move towards strict static typing even in interpreted languages, which I think is driven by actual industry needs and programmer preferences than external marketing, it is becoming a disservice to think of statically typed OO languages as being somehow inferior to the true ideal of object-oriented programming.