Books on Other Kernels
Understanding your enemieserr, competitorsnever hurts. These books discuss the design and implementation of operating systems other than Linux. See what they got right and what they got wrong.
M. Kogan and H. Deitel . The Design of OS/2. Addison-Wesley, 1996. An interesting look at OS/2 2.0.
D. Solomon and M. Russinovich . Inside Windows 2000. Microsoft Press, 2000. An interesting look at a very non-Unix operating system.
Richter, Jeff . Advanced Windows. Microsoft Press, 1997. A thorough discussion of low-level and systems programming in Windows.