It kind off depends on what exactly you don't understand, but a book about Operating Systems and Linux Programming are probably what you want to read. They'll teach you about threads, segfault, the meaning of uninterruptible sleep, signals, the /proc/ directory, etc.

Can you suggest a book about Operating Systems?

I'd suggest "Operating Systems: Three Easy Pieces". (http://pages.cs.wisc.edu/~remzi/OSTEP/)

I've found it very accessible. Largely, the way that they teach is to describe a system based on a set of assumptions, then slowly relax each of the assumptions one by one until you reach an example of a real system. That style of teaching really works for me.

"Operating System Concepts" by Silberschatz, Galvin and Gagne, is a classic. It covers the basics: processes, syscalls, filesystems. (it's what we read in our OS course in university)

Thank you!

A more pragmatic book is The Linux Programming Interface, by Michael Kerrisk. It is Linux-specific though, but that's probably fine.

Uninterruptible process sleep is covered in section 22.3. Threads, in chapters 29–33. Signals, chapters 20–22. The proc file system in section 12.1. Memory mappings, chapters 49–50.

I haven't read that book, but I enjoyed a conference tutorial about cgroups and namespaces by Michael Kerrisk. One of the best trainers/presenters I have experienced!

The version I have covers kernel 2.6. Is that new enough to still be useful?

Useful yes, complete no.

Uninterruptible sleep, signals, memory mappings etc have not changed fundamentally.

Namespaces and cgroups are new concepts in Linux, they won't be covered. IIRC user namespaces appeared in 3.14. New cgroup hierarchy got somehow usable much later than that.

The new concepts are needed for containers. Not at all relevant for the original article here.

Actually he is still maintaining a list of changes since the book has come out. http://man7.org/tlpi/api_changes/index.html

Of course. Linux famously "doesn't break userspace" so almost anything that works in 2.6 still works today.