This also needs to be at the same level as the line above it.

The reason this is important is because you ended up including an extra `end` here, because it looked like a block. As a result, you're missing an `end` at the close of the `Library` class, which means that you also indented the entirety of the `Book` class unnecessarily.

Paying close attention to indentation helps us avoid hard-to-track-down bugs.