I had this working for a split second, then when I cleared the test data, it broke again.
In my Checkout Book > Book Checkout table, there’s a column called Due Date. It’s a formula based date that is the checkout date + 42 days. I have a Checkout Log. It’s pulling in the the other book checkout columns (Book Group, Book, Borrowed Date), but not the Due Date. How can I fix it?
The current code in there is at the “I have no idea why this isn’t working, I’ll ask for help” level.
Hello there @agoodman!
The problem is the paremeters is your lookup formula.
In this formula you don’t use comparison operators, the equal sign is kind of embeded in the formula, so instead of writing
Books.Lookup([Book Title]=thisRow.Book).[Due Date]
you just use the comma
Books.Lookup([Book Title],thisRow.Book).[Due Date]
If it makes more sense to you to put a comparison operator you should use the filter formula that is a lot more flexible. The formula in that case would be
Books.Filter([Book Title]=thisRow.Book).[Due Date]
(I also changed the table where you are looking the book to Books, instead of Book Checkout as you had it, because there the Becoming book is filtered out of that table so it wouldn’t be able to find it)
Now that you understand this I would like to explain why you don’t need any of these formulas in your case 
In the book column, instead of using the book title as text you could change it to a reference of the Books table, that way you only need to put the next formula to get the Due Date:
thisRow.Book.[Due Date]
I leave a doc with this last change: