Two key things before approaching this:
1. You need to tie them to the right metric and measure that. The most common here would be actual signups to your service.
2. Optimizing the content to increase the metric in the above.
To tie it with conversions, a requirement should be collecting email addresses in order to view the white paper or ebook. This way you can track them across the funnel and see exactly which ones converted.
To optimize the materials:
1. Once you are able to track users through the funnel from a tech standpoint, you can easily A/B test multiple versions of your white papers and ebooks to see which work best.
2. Ask the converted customers for feedback a few days after they've already been using your service. A short, direct email will not only give you insight, it'll likely even help you build personal rapport with customers.
Here's how I would word it:
Could I bother you for some quick advice? Your time is valuable, so I'll be short:
What stood out from the white paper I sent you prior to your purchase?
We're a small startup and your feedback on this matter would be of tremendous value to me.