That's a tall order, as server hardware is a bit different from PC hardware, primarily because much of it is in place for years and/or decades. While it might be interesting to follow the latest and greatest evolutions in hardware, as a practical matter, it's often better to focus on server hardware a couple years old.
Also, 'server hardware' runs the gamut from home servers to enterprise servers, to purpose-built servers to one-off custom servers, this is an area where knowing the general concepts is probably more important than knowing a particular piece of hardware...unless you have a particular piece of hardware in mind...in which case it's easy enough to dig up information on that particular application.
I know this isn't terribly helpful, but it's important to keep in mind that some (mostly government backup) servers still use vacuum tubes...because vacuum tubes are completely invulnerable to EMP (electromagnetic pulse).
In other words, if you really want a comprehensive understanding of server hardware, you pretty much have to pick something, and expand outward from there...unless you're talking about servers that are essentially high-powered PCs.