I'd like to recommend just a couple titles...
Looking for fantasy, sci-fi or adventure?
The Pendragon Series by DJ MacHale
The Merchant of Death
The Lost City of Faar
The Never War
The Reality Bug
Black Water
The Rivers of Zadaa
The Quillan Games
The Pilgrims of Rayne
Raven Rise
The Soldiers of Halla
The series starts with 14-year-old Bobby Pendragon and quickly sends him on a fantasy adventure across several worlds most people don't have any inkling of. He's charged with the task of protecting these worlds from Saint Dane. It's great if you're looking for engaging characters, informal first-person narrative and adventure. Lots of adventure.
Ready Player One by Ernest Cline
If you're looking for something a little more grown-up than Pendragon, check this stand-alone first novel out and brace yourself for a total immersion into '80s pop culture. A nerdgasm for those already well-acquainted and prone to moments of nostalgia.
The Three Worlds Cycle by Ian Irvine
The View From the Mirror Quartet
A Shadow on the Glass
The Tower on the Rift
Dark is the Moon
The Way Between the Worlds
The Well of Echos Quartet
Geomancer
Tetrarch
Alchymist
Chimaera
The Song of Tears Trilogy
The Fate of the Fallen
The Curse on the Chosen
The Destiny of the Dead
This is yet another step up, more grown-up than
Ready Player One but still offering the separate worlds, the politics, the adventure. These works are long, but incredibly easy to become absorbed in. Unfortunately, the author is Australian and only the first Quartet was ever published in the United States - and is now out of print, so aside from eBay and Amazon (even they still have trouble), it's incredibly difficult to get your hands on copies, either for yourself or borrowed. It's been nearly a decade since I found a copy of
Dark is the Moon at a Goodwill and I'm still missing
The Destiny of the Dead.
Looking for vampires and other creatures?
The Blood books and the Smoke books by Tanya Huff
Blood Books
Blood Price
Blood Trail
Blood Lines
Blood Pact
Blood Debt
Blood Bank
Smoke Books
Smoke and Shadows
Smoke and Mirrors
Smoke and Ashes
The Blood books follow a human detective, Vicki, and a vampire, Henry. It's a dark comedy and quite the suspense thriller. Each book focuses on a different type of supernatural creature, so never fear, werewolf/mummy/etc fans. Quirky characters, memorable plots and compelling storytelling. The Smoke books follow one of Vicki's contacts, Tony, who turns out to be some sort of wizard. Unfortunately, Huff seems to have abandoned the series in favor of other projects, however.
The Firekeeper Saga by Jane Lindskold
Through Wolf's Eyes
Wolf's Head, Wolf's Heart
The Dragon of Despair
Wolf Captured
Wolf Hunting
Wolf's Blood
A take on the whole "raised by wolves" cliche - and a very well executed one at that. There are two types of species, the simple ones and the smart ones that can talk. The smart ones stay away from man unless absolutely necessary, and man is making it absolutely necessary. A girl raised by the smart wolves becomes the mediator between man and animal, allowing them a tense alliance against common enemies. Tense, political and very spiritual. A powerful coming-of-age story.
Looking for something else but find yourself thinking "TL;DR?"
(In a book thread, for shame!)
The Discworld Series by Terry Pratchett
The Black Jewels Trilogy by Anne Bishop (the whole world of the black jewels, really)
The Vampire Chronicles by Anne Rice
The Cirque du Freak Series by Darren Shan
The Confessions of Georgia Nicolson by Louise Rennison
The Hitchhiker's Guide to the Galaxy by Douglas Adams
The Gunslinger Series by Stephen King
The Ishmael Books by Daniel Quinn
The Artemis Fowl Series by Eoin Colfer
The Earth, My Butt and Other Big Round Things by Carolyn Mackler
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Alright. I'll shut up already.