The Windows Internal books update with every version of Windows and are not retroactive, so you will not see content about Windows XP/Vista on the newest edition of the book. So personally to me the best option would be to get the latest one first to start off with, and then once you've gotten your feet wet a little, you can carry on to the previous versions so you can ascertain the differences between how Windows 7 works internally compared to Vista or XP. The 4th edition covering XP will show you how drastic in difference things are that changed when entering the Vista/Server2008 realm, but you won't notice these differences if you don't already have knowledge about the environment in the first place!
Also, mgrzeg, that's a sweet find, and looks like it got a good review already from Dmitry Vostokov (author of the Memory Dump Analysis Anthology). I'll snatch it up asap. Though despite its young age, I find it very odd that the guy would refer to "rings" for one. Rings is an old, old terminology dated back to the time of WinNT and 98, so that kinda sets a red flag for me. Nonetheless, it's cheap and looks valuable so I'll have a look-see. Thanks!