I started learning kernel internals from these books, I highly recommend all of them.

Operating Systems: Three Easy Pieces (free online) - Arpaci-Dusseau

Linux Device Drivers, 3rd ed (o'Reilly) - Jonathan Corbet

The Linux Programming Interface - Michael Kerrisk

The Design and Implementation of the FreeBSD Operating System (2nd ed) - McKusick, McKusick, Watson

Solaris Internals - McDougall, Richard, Mauro