Accumulating and deleting files in desktop Books app

I use a lot of .epub files for research. I've never known how to proceed within the app after I'm finished with a book, and as a result, my Library has 300 books.

First question: Am I right in assuming this isn't ideal? With that many books, the app seems to take up a lot more memory than normal.

Second question: When you're done working with a book in Books, what do you do? If you delete it, please explain how.

Please note that I'm asking only about desktop functions. Thank you.