Cowtown,
You won't find everything in a single location. You'll need to review the manual thoroughly. But, in the TASO Football Officials Manual for a Crew of 5 Officials, Section 6 - General Game Duties, Article 3, Item A-3 and Item F-2 prescribes the R to identify the tackle on his side of the ball as his primary key.
Section 13 - Forward Passes, Article 1, Item A prescribes the R to maintain focus on the potential passer when he is threatened, and the passer until there is no longer a threat of a foul against the passer.
Section 11 - Plays from Scrimmage, Article 2, in particular, discusses keys for the for the wings and B, in depth, for the passing game.
Generally, the R has the O tackle to his side. The U has the O GCG, and the wing opposite the R has the O tackle on his side (in addition to any receiver key he may have). The wings have their receiver key (and the near O tackle, if opposite the R). The B has his receiver key(s).
Keys are things to check AT THE SNAP, and see what is happening. If there is a threat of foul by/on your key, stay with him. If no threat, find something else to look at. If a pass is released, prioritize the area where the ball will land (but never give up on a situation that may lead to a foul(s)).
If you think there are gaps in the manual, let your District Director know, so he can review with someone on the Mechanics Committee.
Robert