Much of the kernel has changed but a lot of the info on PC hardware is still pretty accurate; e.g. the description of PCI still applies even to PCI-E which is largely backwards-compatible at the software level despite being completely different hardware-wise. (Minor error I noticed in the PCI section: no 8080-based system had ISA, that and the fixed I/O port assignments started with the 8088 in the IBM PC; or more precisely the 286-based PC/AT.)

it's only going to be useful to understand how a kernel works, as an overview. But the specifics (ext2?) are horribly dated.