Can somebody point me in the direction of books, sites, articles, ... to learn everything about Mac OS X "under the hood"? In other words to learn more about the kernel, the system.
ps.: About Snow Leopard of course
Don't overlook http://developer.apple.com - there still is a free developer account option to get Xcode 3. You can also buy Xcode 4 from the Mac App store for $5 and no developer account needed - the public documentation available through Xcode is impressive and comes from the web site. The free Ingredients browser also is superb for consuming and searching the Apple Documentation. You only need to pay for a developer account to get pre-release software and documentation so to learn about the current and past OS is free or nearly free.