Move most of the code into a constructor function.  Add addBook method, getBook method, and change renderLibrary method into a render method in the Library constructor function. Add procedural tests, so they add.
Add checkout book method to Library constructor function that takes a parameter of book title and changes that book's status to checkOut and returns the book object.  If it cannot find the book or it is already checked out then it returns null.
Add a button to your html book template. When the user clicks this button, it checks out that book and updates the page to reflect the new status.