I think it’s intended for novice/intermediate level developers that are ready to really master the core of the language. I’ve had to read the book a few times to really understand some of what Douglas wrote about because it’s so dense.
It’s clear and concise, and there’s a lot of explanations about why this code is better than that code. Douglas has some really fantastic diagrams in the book about how things work.
And finally, there are some pages of the book that are definitely not relevant any longer which I would expect for a book of this age.