When you think about it, don't most books contain an element of every genre?
Romance - not universal, but many novels have romantic subplots or flickers of chemistry.
Mystery - it's hard to think of a book that doesn't have a hidden element of some kind that the reader might try to puzzle out in advance ... even if it's a fairly straightforward work and it's just a question of, "What next?" Fantasy settings have the puzzle of the world.
Science fiction / fantasy - let's face it, even the most mundane book isn't quite like the world as we know it, if only because it's framed and "tidied up" for readers.
I could go on: every book is a suspense; many books include some element of horror, at least in attempting to get the reader to fear for the main character; action is a popular component in other genre works.
Doesn't it come down to which element you choose to make the star?
Random theorizing over. Carry on.