I recommend “Principles of Helicopter Flight” by W.J. Wagtendonk and “The Art and Science of Flying Helicopters” by Shawn Coyle along with the other books listed in the prior posts. Also, pick up the FAA’s Aviation Weather book.
ASA has books and CD’s for the written. I have used the CD’s and it’s good to use them to get the exam out of the way but all you will know is the correct answer to the question without knowing why.
What Flight School are you going to ? Send a message.