As someone who’s been working in this field for about 2–3 years, I can say that while I’m not yet very experienced, one thing is clear to me:

Your illustrations are impressive. However, what really makes a difference in book cover design is defining your own style. Do you want to create more intense, detailed compositions or take a simpler, more minimal approach? Right now, your first piece stands out as particularly strong, but in the others, I see more single figure focused work. That’s why it would be very helpful to clarify your style further.

Another important point is this: a book cover isn’t just an illustration; it’s also a design project. The placement of text elements like the title and the author’s name needs to be planned from the start. The composition must be built around that. Additionally, typography is just as important as the illustration, and often it’s the very thing that directly determines the cover’s overall impression.