The Garlands of Thorn and May trilogy have closely-related covers with the same styled figure and floral motifs inside frames.
The first three books in the Elydian Dawn series have similar backgrounds in differing colours and all feature the same major character.
Then come the three stories in the Court Leopold mini series... all of which feature Court Leopold and his horse, Art.
The fourth set of lookalike covers is the Pearl series.
Then come the Cat Mahal is Magic series (a work in progress)
How do you like look-alike series covers?