I have two observations on crossbow books. First: You can download my primer on medieval crossbows from my website at www.crossbows.net I wrote it 15 years ago and it needs some updating, but it may be useful to some of our company.
Second: Payne-Gallwey's "The Crossbow" has a lovely pattern for a Flemish arbalest, but there are a few things I recommend you do differently. The steel-lined lock socket would be a major obstacle for most craftsmen. You can do very nicely by inletting blocks of moose or Axis-stag horn in the stock and cutting the roller-passage between them with a 'Forstner' bitt. P-G makes his roller a bit shallow, at 3/5 below the top edge of the socket and 2/5 above. I recommend you bury the roller 2/3, with 1/3 protruding. That will give you a much stronger socket. If you bury it as much as 3/4 of it's diameter, you won't be able to take the roller out of the top of the socket.
P-G places the sear-plug, where the trigger/tickler bears against the nut, almost precisely at the bottom of the nut. If you move the sear around toward the rear of the nut by, say 5 mm. You will get a stronger lock, since the trigger takes more load and the upper edges of the socket are stressed much less.
Last of all, P-G shows a trigger with complex angles within a passage with similarly complex angles. I'm not sure you could really fit the trigger into that passage, without going in the side. If you look at ancient bows, most of them feature a simple Z shaped trigger that is fitted into a simple slot in the stock. If you feel the slot is too large and unsightly when the piece is finished, one can always fill the excess space with a block and then inlet a decorative bit of bone or fancy inlay over all. Remember, if you make a mistake, you can usually repair and hide the fact with a bit of decoration. In my shop, we call it a 'feature'.
Other recommended crossbow books: Die Armbrust, by Egon Harmuth, pub. by ADEVA press, Graz, Austria, 1986. A Guide to the Crossbow, by Wm. Paterson, privately published by Society of Archer Antiquaries, 1989. Available through Manchester University Museum, Manchester England.
Thoughtfully yours: Geezer
I hope this helps. Geezer