OS X internals is perhaps the tome-y-ist book we have in objc land. It's hard cover only and about 2 inches thick. It was the book I quoted most in my dissertation on the internals of Mac OS X. And it hasn't been updated since 2006. This doesn't mean the current version is useless, as the Macs internals hasn't changed as vastly through the latest iterations of OS X but I expect the iOSification of the Mac will definitely have had some interesting changes to the whole system.
The book is pretty low level, as it aims to explain every moving part in a modern operating system but is approachable without a raw computer science background IMO. You can give it a taster with this excerpt on "A Technical History of Apple's Operating Systems":
http://osxbook.com/book/bonus/chapter1/pdf/macosxinternals-singh-1.pdfhttp://osxbook.com