Stevens is possibly only really necessary for UNIX systems developers these days, but I see enough people with fundamental misunderstandings about protocols and networking and memory management and processes, etc. that maybe they should also be considered mandatory. At least worth a skim.
Programming is too young of a field to have a "canon". And I think any field that does have one that people really care about is probably a bit ossified and dead.
I really liked The Pragmatic Programmer, although some might think it's not a very technically challenging work.
I also like most of O'Reilly's Perl books.
Also, Jonathan Rockway's book on Catalyst was very helpful for me, although there were some issues with typos in the first edition (which is to be expected.)
Also, Philip Greenspun has published a good free collection of computer-science and web-development related texts on his website.
(Sure, he's a little arrogant and some people find his writing style to be somewhat abrasive or even borderline nonsensical, but I certainly liked most of his work. And I believe I learned quite a bit from it, as well, though I'm sure that is debatable.)
Stevens is possibly only really necessary for UNIX systems developers these days, but I see enough people with fundamental misunderstandings about protocols and networking and memory management and processes, etc. that maybe they should also be considered mandatory. At least worth a skim.