If you already know the basics of what an OS does, it's best to just get into some more detail of how it does it. "The Design and Implementation of the FreeBSD operating system" is a great book for the structure of a *nix system. If you understand the design of one of these systems, none of the others is hard to figure out.
Beyond that, if you want a more academic read it's good to read up on some other architectures. This paper on the history of the L4 microkernel family is a really good discussion of the design decisions and evolution of that system that gets into some classic OS issues: https://trustworthy.systems/publications/nicta_full_text/898...
Beyond that, if you want a more academic read it's good to read up on some other architectures. This paper on the history of the L4 microkernel family is a really good discussion of the design decisions and evolution of that system that gets into some classic OS issues: https://trustworthy.systems/publications/nicta_full_text/898...
Other kernel designs worthy of interest are exokernels and unikernels. Also with regard to systems design I've always liked "end-to-end arguments to system design" (https://web.mit.edu/Saltzer/www/publications/endtoend/endtoe...) and another paper on the duality of OS structures (http://occs.oberlin.edu/~ctaylor/classes/341F2012/lauer78.pd...)