Checking a custom post's taxonomy by slug

I’m writing a custom plugin for my own use. Here’s the setup.

I have custom a post type and taxonomy.

Book Author (custom taxonomy)
– Book (custom post type)

I wrote a script that automatically adds books. However, I don’t want it to add the same book more than once. So, I need to know the best way to check the following expression:

if the book-title and book-author combination both exist, do not add the book. However, if the book-title exists but it’s under a different author, add the book.

See what I’m saying? The same book title can exist as along as it’s under a separate author. I need a way of checking this that is server friendly.

Thanks ?? Let me know if you have questions