Book

Book form is a form especially suited for books. This is the way authors write and speakers speak when they want to convey nothing with regards to formality or position.

If you do have a reason to know your audience, or to know the formality of the situation, this is improper to use.

You'll see book form in textbooks and literature intended for a wide audience. You probably won't find it in letters.

You'll hear teachers use it when making a video for broad display (where the viewer might be senior or junior), but they won't use it if everyone in their class is junior to them.