
I have a blog that I (very loosely) monetize through amazon affiliate links. Often I will reference a book link it in an article. The conversion from clicks to Amazon to purchases is around 1%.
I recently shared data on my email open rates and wrote what I learned from a copywriting book and that article has been converting at around 11%! It's only about $5 that I've made but it is such a good feeling. This is the article -
https://www.residualthoughts.com/2020/08/19/my-email-open-rate-data-and-3-copywriting-tips/
I think the fact that the book is directly useful and not just generally interesting is what is driving the conversions. Something that will immediately help your audience monetizes better than something that you think they'll enjoy.
That said, I really like recommending books I enjoy so I think I'll keep doing that but it's a good thing to keep in mind!