For series, giving every book a title with the same structure can become repetitive and confusing. 'The Copper Cat,' 'The Silver Fox,' 'The Golden Eagle'... by book three, readers can't remember which metal/animal combo they read. It sacrifices distinct identity for brand consistency. Each title should be memorable on its own, not just a slot in a pattern. The mistake is prioritizing a tidy marketing scheme over giving each installment its own memorable name.
I keep it stupid simple. My rule: the title must be a phrase a character says at a pivotal moment, preferably near the climax. It guarantees the title is thematically relevant and has emotional weight within the narrative itself. Readers love that moment of recognition, where the title drops in dialogue and everything clicks.
It forces you to write a line that carries that much weight, which is good writing practice anyway. You don't have to finalize which line until you're deep in the draft, but keeping this rule in mind means you're always listening for it. When you hear the line, you'll know. That's your title.