"Seems contradictory to me."
To say the least!
Are both volumes provided by your company (for use on the flight deck)?
What sort of engines are fitted to your aircraft?
I'm not familiar with the 737-300, but the text for one of the larger Boeing technical manual reads:
"Ground Idle/Flight Idle Select Solenoid
If energised, the engine idle is low (ground idle). If de-energised, engine idle is high (flight idle). The solenoid is always energised (ground idle) in the air or ground mode, except during landing approach..."
Assuming Boeings are pretty much standard, perhaps your second manual has omitted the "except" part?
I wish Boeing would simply stick to two terms. High Idle and Low Idle... and not mention Flight Idle, Approach Idle, Ground Idle, etc.
Regards.
Q.