Super / Ian (the fly nearly made me not reply... I may bill u for a new monitor when I stab my pen through this one! ;-))
The textbook I had in mind was one called "Basic Structures for Engineers and Architects" by Philip Garrison. Its about £23 on Amazon I think. Its nothing to do with scaffolding really but it explains the basic calculations you would use with good diagrams. Its written in a way which makes it easy for people to understand who aren't used to reading that kind of thing.
Other than that, I would suggest reading TG20:08 cover to cover and taking time to digest what it is saying. I know that's a ball ache but as a designer its vital that you can call upon that basic knowledge in any situation.
The other code which is very useful is BS 5975:2008. That applies to ALL scaffolding in the UK, you could design to 5975 rather than TG20 if you wanted, it contains everything you would need as an engineer, its just never really mentioned. If you did read it I am sure you would be amazed at some of the stuff main contractors should be doing for you (as scaffolding contractors) which just never happens.
One thing I wouldn't do at the outset is worry too much about understanding the wind codes - they have just changed to Eurocode anyway. Although you will need this knowledge before you start designing, I would suggest working on the other things above first.
I hope this helps (unlike the fly)
Ben.