I have an email signup offer on my website and used on facebook ads etc at times. The offer is to get a free book library with four free novels and six free short stories.
The novels and stories are a mix of Sci Fi and Fantasy. Two of the books are from the Wanderer's Odyssey series. They tell the story of a young slave called Jess (who's about 16) and how he accidentally becomes linked with an incredibly advanced ship called the Wanderer, which gives him his freedom and sets him on a path of adventure. The ship itself is very futuristic - sleek and seamless, and silvery. Very different from the more functional human ships.
The third book is Dark Soul Silenced - Part One. This is an epic fantasy book set in a medieval world where all magic is viewed as dangerous and evil. The main character, Daniel, has been attacked by a vampire at the start of the book and is starting the transformation into becoming one himself. Due to the strength of his will and a chance encounter he manages to choose death, destruction, over becoming a vampire, but in doing so he survives and the darkness is burnt out of him. He's left with powerful magical abilities which he tries to use for good, but in the world he inhabits that still marks him out as evil. While vampires play quite a strong role in this series it's more of an epic fantasy than straight vampire book. One thing that might be useful to know - the vampires eyes are a deep, burning red but Daniel's are grey, though they can still glow at night.
The final book, Last Sunrise & Other Stories, is a real mix of Sci Fi, Fantasy and more speculative stories, as are the six short stories.
I'm very open to letting the designer do what they think is best. I think it could be good to merge the sci fi and fantasy themes, one to each side and merging together, but I'm not great with creating designs and that might not work at all so please don't feel constrained to that. If there is a straight choice I think the sci-fi theme would be the one to go with as it's the most successful side of my writing and is where I have the most books out currently.
The current signup form and books are here, but it's very basic - just something I got designed on fiverr.