I'm working my way through the eighth edition of "Simplified Design of
Steel Structures" by Ambrose and Tripeny (Parker having died at some
point), and I'm finding it to be an odd mix of very basic definitions
and material that requires substantial background. In the latter
category, they've suddenly launched into shear and moment diagrams
with no explanation of just what the diagram is visualizing.

Can somebody point me at a good source for this?