I've worked in civil service for around 14 years. What particular exam or position are you interested in taking/ applying? Have you applied and are already scheduled to take the exam?
Just as ni8killer says, there are vairious types of civil service examinations. One exam does not fit all. There are competitive exams. Competitive means open to the general public as well as a current civil service employee as long as qualifications are met.
There are promotional exams geared for employees going after a promotion within a civil service company. Promotional exams are generally allowed only to current employees with a few exceptions.
And just as what_say_you says, there are study guides you can get your hands on depending on what exam you will take, and a good place to start is where you apply for the exam itself.
In general you should not have to pay for a study guide. I spent over 10 years helping people get employment in a civil service system. It is not as hard to gain employment as some make it sound.
Civil service exams are constantly changing, and so do the study guides. Get the study guide from the place you apply for the position or exam. If there is no study guide, they can lead you to where you should look. Exams are specific and have specific study material. Books can be used for study material of certain exams. If so, there should be a list of study references for you to follow.