If you've got books on your bookshelf that you want to move into a different library, it's straightforward. Click the Books icon beneath your book and then click Move to Library. You can then choose the library you want to move the book into.

If you want to move multiple books at a time, you need to switch to grid view. Click on the grid icon in the top right of the toolbar.

Then, click the Select button and click on the books you want to move. Lastly, click on the Move link and choose the library you want to move the books to.

As the teacher, you can also move your students' books from one library to another. Just be aware that if you move their books to a library that they haven't joined, they won't be able to see the book anymore (until they join that other library). You'll see a message like this: