Selecting one variant out of different similar results

I have a table that looks as follows:

Book    Format    Contents
A       html      ...
A       markdown  ...
B       html      ...
C       markdown  ...

I want to return only 1 version of a book. If a markdown version exists, I want markdown, otherwise I want html.

I'd love to see a SELECT statement that does this, and also a reason why it works.

I think this query is very close to what I need, but I don't understand how it works.

Thanks!