The audiobook of 'The President Is Missing' read by Dennis Quaid (and others) was a whole production. That kind of star power in the narration draws in listeners who might otherwise skip it. It's another example of how a book's popularity can be amplified by its presentation in other media formats.
Forget series versus standalones. The true top-selling 'book' might be one of the early 'Alex Cross' novels that has been in print for 30 years and sold steadily every single week since publication. 'Kiss the Girls' or 'Along Came a Spider' have a cumulative sales tally that a new release, even a huge one, can't match in a single year. The long-tail sales of those foundational books are monstrous. So while we debate which new release hits #1, the all-time champion is probably sitting quietly in the backlist, selling thousands of copies a week to new readers discovering Patterson for the first time.
My first was '3rd Degree' from Women's Murder Club. A bombing opens the book, and the investigation threatens one of the club members directly. The personal stakes are immense from page one. What I loved was how the friendship between the women was both a support system and a vulnerability the killer could exploit. It has all the investigative pieces but makes you care deeply about the people solving the case. That emotional core is what makes the series so enduring for me.