The genius of the book is that it is at the appropriate level of abstraction for people who are not yet experts in Unix Internals, but who are interested in learning. This is not a book for beginners, but it is an excellent choice for serious students who are trying to become experts.
Even given its age, a lot of the information is still relevant. I have been disappointed that the long-promised new edition has not been released yet, and I even considered delaying the release of my own book until I could consult the new edition. I'm glad that I did not delay my own release, since it appears that the 2nd edition was cancelled.