The current released/formatted version of the handbook is here: http://emc.sourceforge.net/Handbook/index.html
Alternate versions (PDF) and other large EMC related files can be found HERE.
Please keep in mind that the most current files are the one on the source forge.
The handbook is an evolving document that is maintained on the Source Forge site along with the actual program code for the EMC software. You can browse the CVS archives of manual HERE. These files are in the format used by the LYX text editor, but are human readable text if you can endure the embedded formatting strings.
The archive of the old handbook is here: http://www.linuxcnc.org/handbook/index.html
Introduction to HAL (hardware abstraction layer) document: http://www.linuxcnc.org/Hal_Introduction.pdf
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