In addition to reading the exploits of eminent test pilots above, an aspiring tp should arm him/herself with a good understanding of human nature. In particular start with yourself and progress to the interactions with others, the aircraft and its systems, and how all of these might be affected by the test flying situations that you could be exposed to.
In these regards, standard books on human factors would provide a good background, but also some of the lighter texts on thinking, perception, judgement, and decision making. Revisit the concept of airmanship and apply this, and human factors aspects to test flying scenarios.
Modern test flying is much more than stick and rudder skills. It involves understanding how others will see the aircraft or systems, which you will evaluate on their behalf, and how they will react under a range of circumstances; above all, how you will judge these.
Much of this will (should) be taught on the course. The majority involves self learning and developing judgement, particularly of when to apply past experiences. For these I, have yet to find a good book, but look hard in several of the texts above and you should find some good pointers, but this might depend on how they are ‘read’ – understood and applied - by you.