For Linux/Unix, I'm going to recommend a book, which for now is probably too ambitious: "Computer Systems: A Programmer's Perspective" by Bryant and O'Hallaron, a.k.a. "CSAPP".

http://csapp.cs.cmu.edu/3e/perspective.html

This book is targeted at working programmers:

    Most books on systems—computer architecture, compilers,
    operating systems, and networking—are written as if the
    reader were going to design and implement such a system.
    We call this the “builder's persepective.” We believe
    that students should first learn about systems in terms
    of how they affect the behavior and performance of their
    programs—a “programmer's perspective.” 

I'm recommending CSAPP to give you an idea of where the end lies. If you grok its material, you will know more about OS kernels than would be required for any typical front-end or back-end web development job.