Since this is the kernel hackers' guide, and since the kernel doesn't use the run-time library, the fact that Harbison and Steele document the run-time library fully is rather irrelevant for the purposes of this document, even though it is relevant to C programmers in general.
I agree that H&S should be included, but not that K&R should be excluded.
I happen to like K&R and find it easy to read and look things up in; I occasionally supplement it with an annotated (sometimes poorly, IMHO) copy of the ANSI standard published by Osborne/McGraw Hill, ISBN 0-07-881952-90. Even if (like me) you ignore the annotation, it's still cheaper than an official copy of the standard, or was last time I checked. I should probably add it to the bibliography, along with a pointer to the GNU C documentation, since the linux kernel does use a few GNU C extensions.