OSDev.org

I use AThelp. Very cool program and in Czech, very easy to understand language for me. (yay I'm Slavic ) The latest version is from 1997, but it has everything what a OSdever needs: instruction list, opcodes, memory adressing, CPU modes and all about them, interrupts (IRQ and INTs), DOS INT 21h + everything about it like hidden functions, multiplex etc, ROM BIOS functions (int 10h, 12h, 13h, 14h,...), and eventually table of I/O ports like PIT, PPI, DMA 8/16, Sound Blaster, HDD (both XT and AT), FDD, Gravis Ultrasound and much much more. Albeit dated program, but I don't need to OSdev for multiple CPU's or in 64bit and that program is very handy.

My contribution to OSDev'ing is the PDCLib, and for that there's exactly one document I need: ISO / IEC 9899:1999 plus Corrigenda...

I use the OSDev wiki mostly. I'm also never coding without my trusty copy of "C in a Nutshell" beside me.

I use the Intel Manuals, the Wiki, and Behrouz A. Forouzan's TCP/IP Protocol Suite.

Those sum up the critical pieces of text, I have about 25 books currently on my desk and 2 more bookshelves for real emergencies.

Hard to say really, it depends on what I am doing, and since I don't like doing what most other people so I don't use something predominantly. I use many things from that list, intel manuals, wiki (although I link to and edit more than actually grabbing info from it), Michael Abrash' GPBB, VGADoc, other people's code (linux kernel and xorg are favorite), C99 standard, trial & error.

I think in the end I learn most from my own code

Definitely the wiki and the Intel manuals. As you can see, I often use the forum as well

For low level HW related issues the manuals I'm using are ARM Architecture Reference Manual and AT91SAM7X Series Preliminary and for OS related issues this forum, OSDev Wiki and Wikipedia