Best Content Structure Question

I am building a eco-DIY info website. The structure on paper is as follows:

There are multiple topics eg:
– Solar Panels
– Wind Generators
– Insulation

Each topic has it’s own menu of resources including:
– Related Books
– Technical Articles

Each DIY topic is set up as a WP category. I then have Related Books as a custom post type – and am using regular posts for the Technical articles.

The quandary:
Unlike articles there is only one Related Books page for each topic.
My custom menu uses WP_Query to get the Related Books for each category but it seems silly to create a loop as there is only one result.

The question:
Is it appropriate for the Related Books to be a custom post type given there is only one per category – or is there a better way?
And if it is appropriate is there a better way to get the single Related Books page than using WP_Query?

Thanks