Primarily about the history and theoretical basis for the design choices that went into the creation of the Objective-C language, intended for curious programmers.
The original reference work on Objective-C, written by the creator of the language (when the paradigm of object-oriented programming was new), originally published in 1986 and revised in 1991. Consequently, don't expect anything about Cocoa or Mac OS X. Great material if you want to read about the language itself, its history, the motivations behind its design, and its relationship to other languages. (Also good if you want an introduction to object-oriented programming that is not influenced by the subsequent C++ and Java languages.) This is the rough equivalent of The C Programming Language for Objective-C. However, as time goes by, this book has less and less relevance for the typical modern Objective-C programmer, who is almost certainly coding for Cocoa on OS X.
Chapters 1–3 are conceptual, and compare various approaches to object-oriented programming. Chapters 4–8 are highly technical and give detailed information on the Objective-C runtime and class libraries. Also includes chapters on user interfaces (UIs) and language extensions.