What result do you hope to get out of saying anything?

As Joel says, if we know what your goal is, we can better help you accomplish it. What is your intended result: Simply letting the coworker know you're upset? Having the coworker disciplined/fired for creating an uncomfortable work environment? Getting the book voluntarily changed/retracted? Legal action (presumably out of scope for this site)? We can't tell you what you should do, necessarily, but if we know what you're trying to do, we can explain how best to accomplish it.

I want to make sure that this doesn't keep growing and that future incidents like this doesn't happen to me or other people because I didn't speak up. I feel that if I do nothing, I am allowing myself to be abused.

For example, I know that if I simply tell HR how I feel about this, it may come off as if I am complaining about an non-incident. I want to know if there are ways I can channel this.

I'm really sorry that happened to you. I don't know if there's much that can be done from a workplace perspective. You may want to check out Interpersonal Skills for help in how to defuse the situation or confront the author. Remember though, someone's caricature of you is not how everyone sees you, and it's certainly not how you actually are.

" What's the best way to approach this situation?" - the best approach is to keep it to yourself. It's fiction. You are real. Let it go. If you can't do that, be prepared to spend/waste a lot of money on lawyers.

"I had once shared some very intimate and NSFW details with a few coworkers" - assuming the author wasn't among this group of coworkers, have you discussed your feelings with them?

As disconcerting as this may be, I don't see how this is a workplace issue. Even if you could prove that the character in the book is based on you, what could HR do? What could you do? How does this fall into the realm of workplace issues? What you need to do is to contact an attorney regarding libel in works of fiction.

Note that @Elisease is asking an officeplace question, not for legal advice. I see a lot of "retain a lawyer" which is out of scope.

Consider thanking the other coworker who shared the info with you. They are being helpful.

Produce a set of office cartoons using the author as the subject. If they find it annoying they may begin to understand what they did.

For an outline on what not to do unless you want half of the internet discussing whether you're a terrible person, read the recent New York Times article "Who is the Bad Art Friend?".

How about you sue the hell out of this person? Libel? Defamation? Something like that?

"As disconcerting as this may be, I don't see how this is a workplace issue." - One coworker is doing something that 1) makes another coworker understandably uncomfortable and 2) seems unnecessarily targeted. Could this be considered workplace harassment?

@marcelm - the coworker didn't write the book in the workplace.

@mrodo It's a work of fiction? Nobody is gonna take this as a case.

@Elisease How bout a signed copy since the only thing you can do is pretty much let it go.

Was there any indication that the author disliked you before the book came out?

@AlexandreAubrey raises an important question. What was your previous relationship with this person like? Is there a history of animosity? How likely is it that this is deliberately malicious, rather than merely poor taste?

PS I feel like you are going to make this a bestseller, ask for a % for advertising!

in the voice of J. Walter Weatherman from Arrested Development: "this is why you don't share personal details with your coworkers" Ever. Let me repeat....ever. Also - if it was that personal, and not safe for work, you quite likely exposed (no pun intended) yourself to a workplace harrasement claim. Those kinds of things should never.ever. be discussed with a coworker, in or out of the office.