The best way to proceed is to give yourself a task then work through the code to implement it. Some examples:
- Make all author names in the tag browser uppercase.
- Add a line to the main context menu (right click on a book) to change the case of the item in the cell.
- Write a plugin that computes the max of some numeric column then for each book stores the percentage for the that book's number in a custom column.
- Write a plugin that takes a set of ranges along with a label, then for each book looks at the value and stores the correct label in a custom column.
- Write a plugin that tells you how many custom columns are defined of each column type.
And so on. Doing something you are actually interested in is usually best.
Most of us will answer specific questions about how something works or what some API does. Most of us won't answer something like "where do I change the code to do X?" Finding where something happens and what triggers it is your challenge.