Comprehensive, lucid, well-made, and beautifully laid out, it's a delightful experience even if you aren't particularly interested in doing any Smalltalk programming. Really good technical writing has virtues beyond the nit-picky documentation of some particular system. Much of the technical writing I find admirable manages to usefully focus on concrete detail while stimulating a broader understanding. The animal books are cleanly designed, visually comfortable, and often quite useful, but their relentless consistency is a symptom of their failure to be much more than reference works and tutorials. I will not talk about those abominations.
Formality provides several axes of variance as well.
Of course, it does seem to make sense to distinguish between introductory topics and texts and advanced ones, but this division seems to have a different flavor. I've done a fair bit of newsgroup and mailing list technical writing but have recently started experimenting with slightly longer and less ad hoc forms. And it's painful, very, very painful. Which is why I'm writing this article instead of those . I am finding that one kind of good technical article has much of the feel of one kind of good philosophy paper, which is, I suppose, why I like it.