With non-fiction books then the learning and expressing the subject of your manuscript would be essential. A fiction book is more or less the same. Wheather it’s in present-day or a few hundred years ago or in the future, then you need to be aware of currency, eating and drinking material. Off-world Sci-fi or fantasy also needs to be checked as you don’t want characters in a fictional world going for a chicken evening meal and paying for it in Dollars. You must spend time reading outside of your comfort zone, reading the work of authors you admire and the novels of authors you detest.
You must take notes, write down and learn to arrange your ideas before you start your book. I was asked on a writing course to study poetry. My first thoughts were, “Yuck, I hate poetry and will never write any.” But, I considered Shakespeare and looked into his work and discovered he was the Iambic Pentameter master (which is poetry and sonnet material), and that may be a reason his work is still loved 400 years after his death.
If you fail to feed your mind, then don’t expect it to serve you quality ideas when you next sit down in front of the blank page.