The voice of your piece is determined by how you plan to tell the story. In fiction, especially in novels, the voice most often used is third person. The author is invisible and unknown, and the reader is not spoken to in the prose. In many nonfiction books, as in the one you're now reading, the author and the reader are both acknowledged as active participants.
Both types of writing reflect a different voice, because each type of work has a different objective. In a novel or short story, you are trying to paint a picture using words, giving the reader something to enjoy. In nonfiction, frequently you are conveying information or expertise, so the use of first-person pronouns can be completely acceptable. For much the same reason, second-person pronouns are acceptable too, because you're often speaking directly to the reader.
First-person voice won't work in stories in which you want to have scenes that take place away from the main character. Keep that in mind before you choose a particular voice.
First-person novels, however, are a different story. In them, the voice is usually that of the protagonist. There are some exceptions, however; many people would agree that Sherlock Holmes was the protagonist of Sir Arthur Conan Doyle's stories, but the first-person narrator was Dr. Watson.
First-person writing gives a direct involvement with the reader because the reader is allowed to climb directly inside the head of the narrator. Because you're limited to one person, novels written in the first-person voice tend to be easier to write as far as one aspect is concerned. Much of the preliminary work is already done for you as the author because there's no need to decide which character's viewpoint would best describe each scene, since you're only dealing with one person.