I agree. I would place Dijkstra's 'A Discipline of Programming' and Feijen and van Gasteren's 'A method of multi-programming' right up there with 'Introduction to Algorithms'.

What sets these books apart is that they try to tackle the problem of _how_ to design elegant algorithms. In the process they introduce crucial concepts like invariants which, once you understand them, become indispensable tools in reasoning and designing algorithms.

More generally, their stress on elegant proofs develops a taste for mathematical elegance which would serve all programmers well as they design new systems.

These books are much too under-appreciated, in my opinion.