I didn't notice this before, but on commenting on your pictures at PBase, I picked the two pictures you'd posted here. Weird!
Anyway - my children love Terri Pratchet (sp?) and are referring to Discworld stuff all the time. I haven't read his books, but plan to do so next year.
The models pose is very nice but you need some sort of fill light on her face. Either use a reflector positioned below her or use some fill flash (or both). Again the composition is good just needs some light.
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