Singh's book is very expensive - USD $110.00 I recall - but covers a lot of material you will never find anywhere else.
As Alex said, it doesn't cover Intel. The hardware memory management is completely different between PowerPC and Intel, but O'Reilly's Understanding the Linux Kernel has a very approachable explanation of i386 memory management that should be easy to translate into the way the Darwin kernel does it.
The Darwin kernel is a massive fork of the FreeBSD kernel. Darwin has the Mach microkernel statically linked in - not as a microkernel - to enable use of its message-passing, task scheduling and memory management functions in OS X. The FreeBSD driver architecture in the C language has been replaced by the I/O Kit in a subset of C++ called Embedded C++.
Given those caveats, The Design and Implementation of the FreeBSD Operating System by Marshall Kirk McKusick and George V. Neville-Neil may be useful to you. I've read it and found it to be excellent.