Bonus, I'm linking to the Wikipedia or Goodreads pages of all the books, because 1000% recommend all of the below.
Articles of the Federation by Keith R.A. DeCandido is one of the few media tie-in novels that I've read more than once, because though it only features cameos from your favorite characters, it's a really rich expansion of a part of the universe that is pretty much a black hole on screen: Star Trek meets The West Wing, basically. It's set in between Nemesis and the lead up to the reboot, and written by one of my favorite ST novel authors. It's really brilliant, and if you're interested in the broader Star Trek universe outside of ships and the military at all, here's a primer on the civilian political system hidden within a captivating novel.
Midnight's Children by Salman Rushdie was part of a colonialism theme in English class, and pretty much my entire introduction to anything having to do with Indian independence (like, the fact that it happened ? Hello, history classes, what the fuck.) So this was a world-broadening book, for sure, as this historical fiction/magical realism amalgamation required a decent bit of learning about the struggles of newly-independent India to understand both the political allegory and some of the actual plot events therein. Also, Rushdie is partial to the very stream-of-consciousness writing style that I use myself (I was looking over notes for a presentation I had to give on the novel to remind myself why I liked it, and actually had to Google this bit to see if he wrote it or I did; the only hit was my blog pointing it out seven years ago, though, ahaha), so his words flow in my brain like they belong.
Chronicle of a Death Foretold by Gabriel García Márquez has such an interesting non-linear storytelling format that I was super inspired to try it out myself in some writing (which never actually came to fruition, but still). While other things will inspire me in an "oh, I want to fanfic this" way, this was unique in that the story ideas in my head were forming around this narrative technique rather than around characters or settings. I think I've put off my reread in the original Spanish for too long at this point to be able to do it without constant dictionarying, but it's still on my to-do list, and as it's a novella, it might be manageable.
Dark Passions by Susan Wright is another stand-out of the literally 100+ Star Trek books I've read (this is a two-parter, actually), but for less world-building and more "reads like a femslash fic" reasons. This one has everyone's favorite things: it's heavily dominated by powerful female characters, many of whom are queer, and ties the TNG and Voyager characters into the mirror universe established in the DS9 episodes. That's right: (even more) canon. mirror. universe. lesbians.
The Harry Potter series, because of the universe and the fandom and all the magic and memories that entails.
The Sherlock Holmes canon, also mostly for the fandom and the universe and all the fun that has come out of it. I'm one of the newish (though with Sherlock and Elementary, I suppose I'm from an older wave, now!) Holmes fans who saw 09!SH and immediately went home and read all the canon and watched the Granada series and dove straight in to classic fandom while waiting for everyone else to catch up. So even though this was a movie-inspired read, my love for the canonverse is no less strong (and stronger than that for BBC Sherlock, tbh, because I have massive love for Victorian England). Specifically, I love SCAN for Irene of course (here have some meta notes on it from ages ago), DYIN, 3GAR -- okay, the ones whose titles come to mind are just the gayest ones and not the ones with the best stories or anything, sorry not sorry.
The Hot Zone by Richard Preston is a thriller about a filovirus outbreak outside of DC; after a solid chunk of the book on Marburg and Ebola to make sure you're up to snuff on why exactly you should be fucking terrified, we track from patient (monkey) zero of how it got there and how it was contained and hidden from the general public to prevent panic. Oh, and did I mention it's nonfiction? The drama makes it almost novel-esque, though there's enough background on the history and evolution of filoviruses (though from 1995, so I'd need a reread to see if anything's dramatically off of what we've learned since then) via discussion of other Marburg and Ebola outbreaks to make this a solid straddler of the science-read/fun-read category.
Level 4: Virus Hunters of the CDC by McCormick, Horvitz, & Fisher-Hoch is of a similar vein, but more scientific and less dramatized; it's comprised of stories written by the scientists who actually work in the level 4 containment facilities with the incurable plagues. It's a great look inside the lab and was my first picture of medicine that wasn't straight up clinical, so it was really influential in that way. This one was read probably every summer from 1998-2002, and along with The Hot Zone, put USAMRIID squarely atop both my lists of "Places I'd Love To Work" and "Places I Never Want To Be Within 200 Miles Of Jesus God".
Politically Correct Bedtime Stories (and the others in the series) by James Finn Garner is basically what it says on the tin: a satirization of classic stories where the humor is two-pronged -- the narrator is overwhelmingly garrulous in his inoffensiveness while the stories themselves have been updated for a ~modern world and are often changed entirely to be radically feminist or socialist (but I don't recall being angry or feeling poked fun at, because they were so ridiculous?). The Christmas one was once an annual favorite. We spent a week or so reading these aloud in the back of English class sophomore year of high school; they're really funny.
The Hitchhiker's Guide to the Galaxy by Douglas Adams is the funniest shit, eminently quotable, and easily the best six-book trilogy ever published. I haven't read the posthumous one yet, and I hear it's not as good (Eoin Colfer's Artemis Fowl books were great at the beginning and suffered from series fatigue too, so I wonder if you can have the same problem picking up someone else's work), but they're all available on audio, many narrated by Stephen Fry or Martin Freeman, if you're into audiobooks like I am.