More of the same, and that's just fine.
Part Cold-War espionage and spy "thriller," part travelogue, part period piece, part accidental tourist/spy, part gee-whiz-it's-a-series-and-it's-familiar-so-why-not?, part good clean fun (with a little drama and some bite-sized mayhem), and ... well not much else?
Left to my own devices, I'm not sure how much energy I'd put into the series, but ... my spouse (really) enjoys them, and, once we've bought one, it's free (to me), and it's fun to be able to share and discuss books in the home, so I expect we'll keep progressing through the series (although there's no rush)....
Of course, if you plan to try one, do not start here, be sure to start at the beginning. I'm sure they work just fine as standalones, but there's definitely a progression, and familiarity with the characters and the backstory adds color to what, frankly, are rather slender volumes (quick reads) anyway....