My wife recently retired from the USPS and I took the test twice in the mid-90s (and scored really high on my second attempt, but ultimately decided to stay where I was professionally).
Unless things have changed a lot in recent years, the exam book is a great tool in preparing you for exactly what the test will be like, so you can practice before your exam and not be surprised or intimidated by anything. The same book can be used indefinitely if you don’t score high enough to be called the first time and want to try again.
I want to mention that, statistically (at least in New York), about 50% of the people who get the job resign within their first 30 days. This is probably because the work is many times more demanding and stressful than people imagine it will be. I won’t go into specifics here regarding that, but be ready to work harder and probably be more aggravated than you might expect.
Whatever happens, please accept my best wishes for success…