Mental calculus training books

Something a bit more fancy is provided by a company called Art of speed math which also provides a book and training for this.

Other educational/reference books

As you're going to be a trader, it's a good idea to at least read up on the theory of futures and options. It's often not required for a traineeship, but experience learns that knowing this in advance helps a lot.

Stories based upon trading

Alternatively, there are a few pockets about trading you might enjoy reading; stories about the rise and fall of companies and some of the larger trading scandals.