Short answer: the three most frequent mistakes are choosing keywords that are too broad, having no internal links between pages on your own site, and leaving the title and meta description as generic template text.
Trying to rank for "romance novel" instead of a specific angle means competing against publishers and sites with years of authority. The problem isn't the topic — it's the level of breadth chosen.
Every new page should link to at least one or two related pages on the same site. Without that internal linking, Google takes longer to understand which pages matter and how they relate to each other.
Many author sites leave the homepage title as "Home" or the template's default name, instead of using that space (the most important one for SEO) for the author's name and main topic.
Most searches happen on mobile; a site that doesn't display well there loses readers before they read a single word.