UEFI Forum http://www.uefi.org
UEFI Open Source http://www.tianocore.org
EFI Developer Kit (EDK II) http://tianocore.github.io/edk2.html
EDK II Documents http://www.tianocore.org/docs/
UEFI Shell Documents https://github.com/tianocore/tianocore.github.io/wiki/ShellPkg