To add to what Kiwichick said. In my experience, the people that stress out about preparing as much as possible are usually the ones that don't get accepted. Those that do manage to scrape through as a result of their preparation don't make it through the training.
I've said it before, and I'll say it again, because no-one listens
You either have it or you don't. If you have what it takes you'll get in. If you don't, and you get in because you bought books, courses, etc to get you through the selection, then more fool you, because you're going to spend money to move to where ever your training course is, maybe take a huge cut in pay to follow your dream, and then bomb out.
I'm not saying don't follow your dream, but get the job on your merits, your personality, your innate abilities, not those you manufactured from getting coaching from someone else.
Ask yourself, what are you going to lose if you don't get the job? Then ask yourself, what are you going to lose if you get the job under
false pretenses and fail?