Patriot Games easily hooks the reader into the Jack Ryan universe by developing the human qualities of Jack himself.
Years before the events of The Hunt for Red October, and before he joined the CIA, Jack Ryan is vacationing in London when members of the Ulster Liberation Army (ULA) makes an attack on the Prince of Wales. Out of instinct, Jack breaks up the attack. While gaining an ally in the Prince - Ryan becomes the focus of hatred to a new dangerous enemy - one Sean Miller - whose brother was killed by Jack. Ryan must use all of his skills and knowledge at his disposal - and eventually those of the CIA - and battle back against Miller - who has a habit for making things personal.
While most of Tom Clancy's novels are very technical orientated, Patriot Games, however, dwells on the human emotions of passion and revenge above all else. And because of this, we see Jack Ryan as more of an individual then we do in any other story.
Sean Miller instantly becomes one of the most well-written and well-developed villains in the Jack Ryan saga. The ending of this book tends to get back to Clancy's technical style - as he often writes - but the action is extremely easy to follow.
If you are interested in Tom Clancy books - but are intimidated because of his endless details and large number of pages using small fonts - then you should start here first. Not only is it the first chronologically, but it is the least intimidating place to start. Patriot Games is more of a human story than a "Military Scenario" or a "Guide to a Submarine" or a "How to Build a Bomb in Three Hours" type novel. The reading here will be read at a fairly fast pace. In fact, if you have read this book - and liked the style - then I suggest reading Without Remorse also.
Side note: The film adaptation leaves a lot to be desired. On its own, it was entertaining. In comparison to the book - and because they changed so much of the storylines - it was greatly overshadowed. On the bright side, most of the actor's portrayals were very good and right on target. Sean Bean as Sean Miller was a perfect choice.
So, again, if you are new to Tom Clancy, start with Patriot Games and you will be in for an excellent story.