Been thinking about this exact thing myself, since the series is kind of a puzzle. 'The Daughter of Evil' is the main series, with four volumes. You absolutely have to start with those. They lay out all the foundational lore about the Elphegort royal family, the Black Magic, and the tragic story of Allen and Riliane. The 'Miss Devil' books, specifically 'The Daughter of Evil: Miss Devil' and 'The Daughter of Evil: Miss Devil -Green Version-', are spin-offs. They focus on Germaine, the 'Devil', and they take place after the main series events. So reading order is pretty straightforward: main series first, then the Miss Devil books. I tried jumping into 'Miss Devil' once out of curiosity and was completely lost on who half the characters even were, so trust me on this sequence.
There's also 'The Daughter of Evil: Praeludium of Red', which is a prequel. Most people recommend reading it after the main series too, since it hits harder when you know how everything ends up. The way the narrative voice shifts between books is fascinating—from the almost mythic tragedy of the main series to Germaine's more cynical, pragmatic viewpoint in her spin-offs. It really enriches the whole thing if you follow the publication order.