There are 2 ways to add a book to your queue, depending on where you are on the site:
If you are on Your Personal Queue page:
1. click on the "Add books" button
2. Type in the book you want to add
3. Click "Select this book" button to the correct book
If you are browsing books on the site and find a book you would like to add:
1. Click on the box next to "Add to My Personal Queue"
2. Click on the "Add this Book" button and the book will be added to your queue