Without Remorse is, chronologically, the first book in the Jack Ryan series. It's fortunate that this was not the first book Clancy published, because he would have had a hader time topping it with his following novels. It's a tough call, but I'd say that Without Remorse edges out Rainbow Six as my favorite Clancy novel.
The story begins with John Kelly, the former Navy SEAL, who has to endure the death of his wife in a car accident. A year passes, and Kelly falls in love with a drifter named Pam. But it turns out that Pam is a prostitute, and her pimps kill her and wound Kelly.
After Kelly recovers, the CIA asks him to take part in a daring raid on a secret Vietnamese POW camp. Can Kelly balance his desire for revenge and still accomplish his mission with the Agency?
The book is fashioned after a classic revenge tragedy, with a twist. Readers may be shocked that they delight in reading the scenes that describe Kelly's revenge. But that's Tom Clancy for you--he really draws the reader in and allows them to associate with the hero through his effective writing.
Without Remorse is fiction at its best. It's not really a "techno-thriller" like the rest of the Jack Ryan books, but it establishes a framework for the stories to come.