I Am Dracula by C. Dean Anderson is an interesting origin story.
Dracul is a good prequel.
Dracula's Death is a novelization of a lost silent movie which may have been the first film appearance of Dracula.
I have, but have not read yet Dracula the Undead by Freda Warrington (not to be confused with the Dacre Stoker book)
Another one I have but have not read yet (I need to get busy) is Vampire by Hanns Heinz Ewers
If you want to have some fun, read the published shooting script of Ed Wood's The Vampire's Tomb, which he never filmed (Lugosi died just as filming started). If you really want to have some fun, watch the movie adaptation by director
Andre Perkowski. It's a hoot.
If you like nerdy, derivative, fan fictiony stuff, there are these:
Return of the Wolf Man. As his starting point, the author uses the end of Abbott and Costello meet Frankenstein(!), then picks up 60 years later. Larry Talbot comes back from the dead to take on his old foe, Dracula.
The Devil's Brood and the Devil's Night are sequels to the above written by a different author. Dracula's Daughter figures prominently.
Dracula Asylum is a sequel to the Lugosi movie.