Virtual Light by William Gibson.
Chevette Washington is a bike messenger in San Francisco. Not motorcycles, you understand, strictly pedal power. After dropping off a delivery in a high-rise block in the city she drifts into a party on the next floor, where she gets hit on aggressively by some fat middle-aged guy. As she leaves she takes, on a whim, the intriguingly-shaped package from his jacket pocket.
Berry Rydell is an ex-cop working for private security firm IntenSecure. After IntenSecure are obliged to "let him go" following an embarrassingly high-profile bungled bust in Los Angeles, he ends up working for the mysterious Lucius Warbaby in San Francisco. Warbaby runs his own more clandestine security operation, and is investigating the gruesome murder of a fat middle-aged guy who "lost" a vital package that he was supposed to have been couriering between certain shadowy organisations.
I'm sure you're keeping up, so it'll be no surprise to you when these two plot strands come together and Rydell finds himself assigned the task of tracking down this bike messenger who was in the building on the night the package went missing and remains unaccounted for. It turns out one of the reasons Chevette remains unaccounted for is that she lives in a bizarre shanty town constructed on the suspension sections of the San Francisco-Oakland Bay Bridge. We're in a world where the long-anticipated Big One has already happened (cutting off the bridge from through traffic); we're invited to infer that a similar seismic catastrophe (colloquially referred to as Godzilla) has befallen Japan as well. In addition, mutant strains of HIV have killed millions, but the virus is now under control after a particular mutant strain carried by a male prostitute called JD Shapely enabled a vaccine to be created. In the wake of this, and Shapely's subsequent assassination, a bizarre religion has grown up around his memory - well, no more bizarre than any other religion, I suppose, actually.
Anyway, Rydell manages to track Chevette down, but his conscience is troubling him regarding the contents of the package (a pair of "virtual light" sunglasses, which project images straight into the optic nerves), the punishment meted out to the original courier for his failure to deliver, and what might happen to Chevette should he hand her over. So when an opportunity presents itself to do a runner with her and the glasses, he takes it. Needless to say those seeking to recover the glasses are not best pleased and various goons are dispatched to bring them back. Having seen off the first one by giving him a sneaky drug overdose in a can of Coke, Rydell decides he needs to fight back - luckily he has a few contacts who put him in touch with the Republic Of Desire, the mysterious cyber-terrorist group who were responsible for the hacking that resulted in the incident which got Rydell fired from IntenSecure in the first place. It seems the hackers are very interested in the data on the glasses (plans to level and rebuild San Francisco using nano-machines, since you ask, but this is pretty much an irrelevant plot MacGuffin at this point) and might be interested in helping make Rydell and Chevette's problems go away.....
Gibson is the man who made science fiction cool again in the early 1980s as the figurehead of the cyberpunk movement, in particular with his 1984 novel Neuromancer, which you should read almost immediately if you haven't already (you can finish reading this post first, if you like). There was a lot of computer stuff in that (including the coining of the now-hackneyed terms "cyberspace" and "the matrix", as well as one of the most famous opening lines in modern fiction), something there is surprisingly little of in Virtual Light. In fact what this most closely resembles is a futuristic Elmore Leonard novel; Gibson has the same economical way of sketching characters and rendering dialogue.
So is it science fiction? Well, I guess the answer is: it only matters if you imagine that there is a rigid dividing line between science fiction (or what JG Ballard used to prefer to call "speculative fiction") and "proper" fiction. Is Pride And Prejudice science fiction? Well, it depicts events that never happened. OK, let's restrict the definition to events that could never happen, or ones where the surrounding world is unfamiliar to us. Well, what about Doris Lessing? What about Inversions? What about Nineteen Eighty-Four? You see how pointless these distinctions are. What is far more pointy is that this is a highly enjoyable futuristic thriller with some typically sly observations about modern culture and where it might go next, not as revolutionary as Neuromancer - and I'd still recommend reading that one first - but none the worse for that.