html/latest - Kernel Documentation (mainline)
html/next - Kernel Documentation (linux-next)
README various README files scattered around Linux kernel source
Single Unix Specification v4 (Also known as Open Group Base Specifications issue 7, and POSIX 2008. See especially system interfaces)
C99 standard (current version of the C programming language): ISO/IEC C9899 PDF, html, or searchable website.
LP64 standard defining the size of char, short, int, and long on 32-bit and 64-bit platforms. (See also the rationale for the standard, and the legacy reasons another OS declined to adopt this standard).
Linux Foundation's specs page (ELF, Dwarf, ABI...)
Development mailing lists available on kernel.org
All Linux Weekly News kernel articles indexed by topic
Linux kernel changelog (summary of what's new in each version)
H-Online's Kernel-Log (most recent first)
Linux Device Drivers book (third edition) (second edition)
Understanding the Linux Virtual Memory Manager, online book by Mel Gorman.
What every programmer should know about memory, article series by Ulrich Drepper, parts one, two, three, four, five.
Ars technica ram guide, article series by Jon "Hannibal" Stokes, parts one, two, three, Cacheing, Bandwidth and Latency.
If you have any questions or concerns, please email helpdesk at kernel dot org.