Here's how "based on" the book the movie is: Both lead characters are former US Navy SEALs who begin the story with the name John Kelly, who end it with the name John Clark, and who have love interests by the name of Pam.
That's really pretty much it.
Which is not to say that Without Remorse is a bad movie. I liked it, despite several typical action movie suspension of disbelief problems. Example:
* SPOILER ALERT *
Kelly and some other SEALs endure a transport aircraft shootdown (by a Russian fighter) into the Barents Sea (average water temperature in its warmest area -- 40 degrees Fahrenheit) and not only survive, but subsequently waltz into the Russian port of Murmansk in not-quite-broad daylight aboard a Zodiac raft.
* END SPOILER ALERT *
That kind of thing.
I find the choice of story change ... odd.
The novel seems tailor-made for a film adaptation catering to today's "human trafficking" moral panic.
That whole story line -- the MAIN story line -- is absent from the film.
Instead, the film starts off looking like it's going to be 100% post-"Hillary-lost-because-PUTIN" Russia-baiting bullshit.
Fortunately, it rises above/beyond that, and even makes a good (and very non-Tom-Clancy-esqe) point about the actual nature of things near the end.
I don't want my 110 minutes back or anything. It was a fun ride, and presumably will tie into the Amazon / John Krasinski reboot of the "Ryanverse" if that reboot can recover from a disappointing second season. It sets the tie-in up pretty well, especially if can keep Michael B. Jordan on board in the Kelly/Clark role.
Personal jinx moment: Very early in the film, Tamara and I simultaneously asked the same question -- "what's a leppo?"