Every book, fiction and nonfiction, is written from a point of view (POV). Most nonfiction books take a first person or second person POV. Fiction books tend to be split, with trends cycling between first person and third person.
I notice most amateur writers use first person POV because they think it's easiest. In some ways, it is because they need only focus on the perceptions of the protagonist, the character narrating the story. That gets a little more complicated with alternating first person POV, popular in romance, especially in the "New Adult" sub-genre. In alternating first person POV, each chapter comes from the "voice" of one of the protagonists. I recently read The Pilot and the Puck-up by Pippa Grant that made excellent use of alternating first person POV. I also can't remember the last time a book had me laughing so hard. The benefit for the reader is a deeply intimate immersion into the character's personality and thoughts.
Books written in the second person POV tend to be conversational, the protagonist "speaking" to the reader. This becomes difficult in novel length work, but authors of self-help books find it useful in addressing their audience directly. It lends a sense of intimacy between author and reader.
I actually prefer using third person POV. This viewpoint can be broken down into two basic categories: omniscient and limited omniscient. I use the latter, giving readers insights into select characters thoughts, motivations, and personalities. Not every character needs that level of development. The peril of using third person POV is jumping from one character's head into the next and the next and so on without adequate transition. Readers today decry what they label "head hopping," although venerated authors like Georgette Heyer did it and no one complained. I figure that as long as the reader understands whose head we're in, then it's not necessary to split chapters among characters. After all, characters don't hold their thoughts and opinions to themselves until a particular chapter has concluded.
Regardless of whether the author uses first person or third person POV, the upshot is that the author must speak from that character's viewpoint and immerse himself within that character's personality. If there was ever a formula for mastering split personalities, then fiction authors use it. When writing, we speak with voices not our own, we see through eyes not our own, we experience emotions and internalize motivations not our own. In short we share the imaginary bodies and minds of the characters we create. Those characters become real, oftentimes more real to us than actual people because they manifest in our minds and take up residence. They're always with us.
It's my guess that ventriloquists suffer ... er ... enjoy the same multitude of personalities living within them, too.