The Art of Unix Programming is, undoubtedly, required. It tells a lot about the Unix way and is still relevant today. Actually, one day I wrote a composition in English about this book at school (which is Russian) and got a really good mark for it haha.

Elegant Objects by Yegor Bugaenko is an eye-opening book about OOP. It's quite fun.

The best learning resource is, of course, documentation. Whenever I'm learning something new, I jump to documentation as soon as I'm done with those tutorial articles.