KP, increased workload, #1.
Operations with auto flight have changed beyond recognition with increasing technology, particularly flight management.
Similarly with FD + AT, which many people mistakenly interpret as manual flight - to improve 'skill'; whereas reality is increased workload due to manual + flight management, same drills, calls, etc, to use technology.
Manual flight without FD and AT should refresh some of the basics of flying, differences in aircraft feel - inertia, and most important which are the important parameters to monitor; pitch, speed, power, and their interaction, improving the feel and anticipation of the aircraft response, … , but not requiring the same level of flight management, no need for drills, calls, etc, improving skills which should help during automatic operations.
Unfortunately many operators and regulators fail to appreciate these aspects, distant from the reality of operations.
Part of the problem stems from renewed focus on risk management. 'Corporate' risk management is more about the management's exposure to risk. For the crew, this means more procedures and checks, and greater responsibility for the day-to-day risk management.
Little of this directly improves safety, but often increases workload - ' follow SOPs' - you will be safe; the expectation that crews will manage uncertainty as if it were known, because of drills and checks.
The 'back to basics' are not the same basics as in training aircraft, but the those of flying current large aircraft. How to apply the ab initio skills on the line; the basics adapted to your aircraft.
Books:- not 'how to fly', but an updated version of 'Handling the Big Jets', manual flight with modern control systems. Additionally a book on 'Managing modern aircraft systems - flying', but written for management and training, concerning workload and HF, drills, calls, procedures.