"Say hello to x64 Assembly, part 1"
(Incidentally, I did a fork of the early stuff just to see what it would look like with gas syntax - the standard for gcc. Now that there's more code in the parent repository, maybe it's time to revisit):
Another quick intro to 64-bit assembly:
A book (I haven't read), but looks ok, from a quick glance at the examples:
For a long time, the best resources were (arguably still are) for 32 bit x86 assembly -- but IMNHO it's rather unpleasant in terms of segmented memory, limited registers, and the C call abi is also a bit "clumsy" if you want to work in assembly as much as possible (again, this has to do with limited registers). That said, Randall Hyde has some great resources on 32 bit assembly:
An older resource, for 32bit assembly:
I have an old print-out of this on my bookshelf - it's a pretty straight-forward introduction to how one might go about to combine C and assembly. It'd be great to see an updated version for 64bit though.
An operating system in assembler. 32bit version GPL, the 64bit version is as far as I can gather under a more restrictive, source-available license:
Then there's BareMetal Os (I couldn't find any License.txt):
There's also a wiki on OS dev: http://wiki.osdev.org/X86-64
I would recommend going through the first tutorial listed above, to get a feel for things. Assembly can be a lot of fun - but I think you'll in general will have a hard time doing better than gcc/clang if speed is your goal. Still useful for stuff like boot loaders and such. If you want to play with OS/boot-loader development, it is easier now than ever before thanks to emulators like Qemu: http://wiki.qemu.org/Main_Page