Trying to piece together the Marcus Ryker order drove me nuts for a while—Vince Flynn's estate and Kyle Mills handled things a bit unconventionally after Flynn's passing. The publishing order is the definitive sequence, but it gets messy because the timeline jumps around. You should start with 'American Assassin', which is technically a prequel about Mitch Rapp but introduces the CIA world Ryker exists in. Then go to 'The Survivor' (Mills' first solo book continuing Flynn's outline), followed by 'Order to Kill', 'Enemy of the State', 'Red War', 'Lethal Agent', 'Total Power', 'Enemy at the Gates', and 'Oath of Loyalty'.
I made the mistake of reading in strict chronological order by story date once, and it ruined the reveals about how certain alliances formed. The character development in the publishing order has a logic the other sequences lack, especially regarding Ryker's motivations and his complicated relationship with Irene Kennedy. Reading 'The Survivor' right after Flynn's last book feels jarring but necessary—you see Mills finding his footing with the character, and by 'Red War', the voice solidifies into something that honors the original while moving forward.
The political thriller elements build on each other, so skipping around means you miss subtle callbacks. That scene in 'Total Power' with the grid attack hits harder if you remember the infrastructure threats hinted at in 'Enemy of the State'. Just stick to release order; the timeline sorts itself out.