Speaking as someone who once spent a month trying to make
Payne-Gallway's conception of a set trigger actually *work*....
He's OK for the outsides of bows, assuming you have access to such
everyday things as a local Liege spring maker, but don't trust him for
a second when it comes to either the inner workings of crossbows and
their parts, or to the exact reason why things might have been done a
certain way. Near as I can tell, he looked at the outside of a number
of bows, and based his conclusions on external examination only.
Harmuth actually x-rayed period locks, so his mechanical information is
vastly superior. Alm also has much more concrete grounding in the what
and whyfors of crossbow construction.
For whatever that all's worth.