These are the manuals I use for our Kuka KRC2 with software version 5.2.
Configuration Manual - for those starting out installing a new KUKA robot.
Expert Programming Manual - in-depth, very good if you want to write your own SRC programs from new.
System Variables manual - very long but extremely comprehensive
Serial Read / Write manual - useful for understanding RS232 communication
KRL Reference Guide v4.1 - all KRL commands listed alphabetically (supplied by Jimbo )
Please note - these are KUKA supplied manuals, they are NOT written by me!