Surprised not to see "Design and Implementation of the FreeBSD Operation System" mentioned yet (http://www.amazon.com/Design-Implementation-FreeBSD-Operatin...).

McKusick's 'Kernel Internals' class is based on this, and is well worth your time (though it's a bit pricey to purchase the videos on your own: https://www.mckusick.com/courses/advorderform.html)