This page lists various websites which I believe are very useful for insights into the OS X and, particularly, the iOS kernel, also credits.

Amit Singh's Book (OS X) - Amit Singh wrote the de facto bible for OS X back when it was mostly PPC based. His book - "Mac OS X Internals, a Systems Approach" is, by now, somewhat outdated, having been written around the time Tiger was released. It still remains, however, a valuable compendium, delving deep into the implementations in XNU. Some people apparently criticise it (and my book) for having too many code dumps, and state you can just read the open source yourself. Unless you're eager to read >700,000 lines of code that could be structured a *lot* better, I say - get the books.
The iPhone Wiki (iOS) - The iPhone Wiki is a free and open Wikipedia style repository, started by noted iPhone and PS3 hacker, GeoHot. Since then, it has taken a life of its own, with many contributors (including the author).
GeoSn0w (@FCE365) has built this beautiful website :P
Mach publications - CMU still provides published and unpublished papers on its kernel, Mach, which serves as the foundation of OS X and iOS's XNU. While some are obsolete, these are useful references for anyone wanting to know more about the inner workings of Mach, and therefore XNU.
Hacking and Securing iOS Applications: Stealing Data, Hijacking Software, and How to Prevent It - An interesting book on various iOS Ramdisk hacks, and other attack methods