Can you recommend any books in any genre set online

The Otherland quartet by Tad Williams (first book: City of Golden Shadow) takes place largely online in a near-future world.

Killobyte by Piers Anthony featured what's essentially a virtual-reality MMORPG, long before the term existed.

ETA - While I haven't read it yet, Erebos by Ursula Poznanski features on online game with possibly sinister motivations.

Erebos is pretty good. Epic, Saga, and Edda by Conor Kostick are really exciting books explaining economics via MMORPGs. For the Win by Cory Doctorow is great as well. True Names by Vernor Vinge. And Codex by Lev Grossman. And The Rapture of the Nerds, also Cory Doctorow. I like books about video games much more than I like actually playing video games

Some I haven't read: WWW:Wake, This Is Not A Game, and some Charles Stross.