I am currently reading two Linux books from O'Reilly: Understanding the Linux Kernel and Linux Device Drivers.
It's not that I am using Linux intensively at the moment. I have SuSe 8.1 installed in DualBoot mode, mainly for playing around with KDE and OpenOffice, and for experimenting with Java under Linux. And I run the Visual Chat Webserver on Red Hat. But I am far from being a Linux expert.
No, but those books attracted my general interest in modern operating systems. While I know I will never be working on the actual implementation of an OS (actually Windows NT and Linux might have been the last time that any microcomputer operating systems had been designed from the scratch - yes Linux runs on zSeries today as well), and probably also won't implement a device driver in the near future, it's still great fun to read those works - more than purely theoretical ones (e.g. Andrew Tanenbaum's "Modern Operating Systems" certainly is a great book, but just not that kind I will finish at once during a reading week. It serves me well as my primary reference, though).
I'm actually starting to dig into the Linux kernel code for the first time (some of my colleagues at college compiled their own Linux kernel just for the fun of it - that was back in the mid-nineties). I might as well have started reading about Windows kernel internals or programming Windows drivers. While there is plenty of literature about these topics, I doubt that those books describe the reasoning behind the system designs in such great detail.
When you choose a subject because of personal interest, it's just much more likely you get into a reading flow. That's unlike other stuff that I must acquaint myself with, e.g. because of work. I bought three books about Service Oriented Architecture lately in preparation for a SOA workshop that I am going to attend, but the topic is quite dry, and I am still struggling with the first chapters.