My second thought on a set of three books was Virginia Andrews Flowers In the Attic series. There’s five books in this series and a variation of different covers for each. The books are dark and the covers of my copies (above) suggest thriller/horror novels.
Edward Gorey creates fine lined, dark, aerie illustrations controversially used in children’s books. Both sets of books I have contemplated so far would suit a darker style of drawing/typography and Edward Gorey was my initial instinct for inspiration.