This is undoubtedly one of the finest computer books I have ever owned. It is readable enough to use as a textbook, but well-indexed and complete enough to use as a reference. I bought my copy in 1991, and it is almost falling apart. Colleagues are constantly borrowing it, and they usually go out and get their own copy (after I tell them that they may NOT take it home). When Advanced Programming in the Unix Environment came out, a co-worker and I both decided that we would, on faith, "plunk down the 50+ bucks to own it, too". It has also proven to be readable and useful. I took a week-long class in network programming shortly after I bought the Unix Network Programming book, and I took it along, thinking I might need it. The instructor saw the book, picked it up and said "This is THE definitive reference on network programming in Unix. Any problem I have had, I have been able to find a solution in this book!" (All of this was before Steven's series of 3 volumes came out).
Now, if you'll excuse me, I have to go arm-wrestle one of our student interns for my copy of Unix Network Programming...