Book note
Book note is a type of note which displays all its children's content in a linear fashion, so you can read note subtree like a book. This can be useful for reading many smaller notes. Seeing an example might explain this concept better:
Notice in the tree on the left pane how "node.js" note has some children and these are then displayed on the right pane.
You can also switch to "grid view" in the Book properties:
