Background
As your WordPress website grows, you might want to add more than just “Blog” articles and “Pages”. You might, for example, want to add Books, or Knowledgebase Articles, or even Products.
Our recommendations
In general terms adding add-hoc new content apart from Posts and Pages can be done using what’s called “custom post types”. A plethora of plugins exist to help you with custom post types. Vander Host recommends “Custom Post Type UI” or “Advanced Custom Fields / ACF Pro“. If, however, you would like to review the possible types, see the following article:
7 Best WordPress Custom Post Type Plugins: How to create custom post type
Within that article is a really useful spreadsheet with extremely detailed information, see here:
https://docs.google.com/spreadsheets/d/1mSqienVYxLopTFGLPK0lGCJst2knKzXDtLQRgwjeBN8/edit#gid=3