Seriously, I've trawled MSDN and only got half answers - what do the columns on the Task Manager mean? Why can't I calculate the VM Usage by enumerating threads, modules, heaps &c.? How can I be sure I am accurately reporting to clients of my memory manager how much address space is left? Are their myriad collisions in the memory glossary namespace?
An online resource would be most useful in the short term, although books would be acceptable in the medium term.
Try the book "Windows Internals" by Mark Russinovich and I think some other guy too. It's pretty good on getting down to the nitty gritty.
Here is a quick article on Windows Memory Management, which goes into sufficient depth to interpret what you're actually seeing in Task Manager or Process Explorer.
Mark Russinovich has written the excellent book Windows Internals. A new edition that covers the Vista and Server 2008 operating systems is currently in the works with David Solomon, so you may want to pre-order that if your questions are about the new Windows operating systems instead of the old ones.