If you are dead-set on pilot training as your end goal, and don't particularly plan to work in Engineering, then frankly I'd go for whichever you'll enjoy most - it's unlikely to make much difference.
Avionics will involve more maths, electromagnetic theory and computer work, Mechanical will involve more in the way of structural analysis and wind tunnel testing. Both, if you fail pilot training, will offer plenty of career opportunities so you might like to consider the issue in that context.
I wouldn't worry too much about books now, they'll tell you soon enough what they want you to buy for the specific courses. Having said that, whichever you go for, I'd recommend getting a good "Maths for Engineers and Scientists" book and working through it, that'll help you through whichever course.
G