Look into a bit of software called Calibre.
Calibre is the heart and soul of my library management system. I couldn't manage without it, and I have studied it and learned it enough now that I can probably use about 20% of its capabilities.
The lookup depends on it finding the isbn # inside the book
That's a bit of a problem. The metadata of many of my ebooks does not include the isbn#.
Don't know if it can manage audiobooks, never tried.
Audio books are not a problem -- I have all their metadata set up and updated correctly, including the synopses that are kept as comments in the book title cell of the index. These synopses were relatively easy to acquire but the eBook synopses, not so much.
This post lists many sources for book metadata.
Some of those sources look useful, I'll explore them. Even if I have to Google search each book individually by author/title, I can still plow through 100 or so a day before my eyes start to cross, so I can finish the project in about a month.