Gavel
The gavel pictured on The Reversal's cover is symbolic of the law and of justice, particularly the prosecution of Jason Jessup and justice for Melissa, Jessup's victim.
The Lincoln
The Lincoln car featured prominently throughout the novel is symbolic of Mickey Haller (who becomes synonymous with the car) and Mickey's itinerant nature. Throughout this novel, as well as the rest of the novels featuring Mickey Haller, he travels from courthouse to courthouse, working cases and just driving to clear his head.
Police Badge
Bosch's police badge is symbolic not only of his position within the police force, it is symbolic of the law, justice, and the reckoning that is coming for Jason Jessup.
Melissa's Dress
Melissa's dress is symbolic of a new life for Jessup (he gets a new trial as a result of DNA that came from Melissa's dress), as well as the profound abuse Melissa and her sister Sarah had to endure by their sadistic step father.