There has probably been a bit of confusion with the OP and DAR regarding the size of trucks and it is understandable. I have spent time driving a 12t LGV cat C but it looked like a 7.5t truck, of which I have also driven quite a few too.
I actually trained on an 18t curtainsider rigid for my C and that was quite a lot bigger than what I was used to, but I soon picked it up. I do find that when I’m driving them, the size is sometimes insignificant because I am seeing things based on what is in the mirrors or in front of me.
As for lining up on junctions, your trainer will show and advise you. For gears, on an 8 speed truck you will find that the lower two gears are often for pulling off loaded or for slow speed, as well as that you may also have a crawler gear. It is advisable not to push cars out of the way with this slow powerful gear as you’ll never pass your test and just annoy your trainer, however much it seemed like a good idea at the time… 
You will also learn such things as block changes and the like. In this time before your training the one peice of advice I can give, well, two peices actually is this.
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Read the road conditions, signs and markings as you are driving around in your car. Don’t just glance at them, take time to go over in your head what they mean. Seriously, because in a truck you need to understand and plan for the size of the vehicle.
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Check mirrors more. Like, every few seconds. The DSA book states to use the mirrors in the following situations:
Moving off, signalling, changing direction, turning left or right, overtaking or changing lanes, increasing or decreasing speed, stopping as well as before opening your doors.
You also need to check your mirrors when stationary (stopped, rather than the writing papery kind) to ensure that no numptie is doing silly things, like a cyclist moving up your inside and leaning on your lorry whilst waiting or hiding just out of your blind spot.
It may seem like a lot but the preparation you can do now will help you down the line.
So the books you need are the ones you probably have for your Theory & hazard perception test:
The Official DSA guide to driving goods vehicles.
Highway code
Know your traffic signs (I got this one as an extra and it did help with explaining a few things). Amazon is your friend here too as it makes life easier 
Moving on. Mr Smythe does residential courses too where you’d be stopping at the hotel up the road for the duration and it does make life easier, but you have now gone and booked so good luck with that.
Put some work into it and whilst you are worrying about the size now, just take it easy. Read up, observe, take time, observe, mirrors and all that. I’m sure you’ll get there like the rest of us…
However, when you go for your C+E I’d recommend Mr Smythe, and he knows it
