The Enneagram is a description of nine personality types.
1. The perfectionist
2. The helper
3. The achiever
4. The romantic
5. The observer
6. The loyalist
7. The epicure
8. The boss
9. The peacemaker
Presumably most people belong to one major personality type, with a slant towards the personality type on either wing. For example if you are a romantic (type 4) you may also have some elements of the achiever (type 3) or the observer (type 5). However most people would be able to claim to have some traits of each type of personality, and no one can be simply classed into a single type.
The book is not simply a description of the nine personality types. Each type has nine different levels, depending on how the person develops and uses that personality trait. For example, a perfectionist may be in a healthy level, using this trait to apply himself meticulously to his job and achieving the perfect result. On the other hand, at an unhealthy level, the perfectionist insists on perfection at the expense of everything, and may never produce a piece of work because he is never satisfied with it. This shows that a character trait can have two sides and is a helpful tool to guide you make the most out of your personality.