I have always loved to read, though I haven't been able to keep up like when I was a kid. Because of this, I'm often nostalgic for the older series. My favorites have always been Fantasy and Science Fiction, with a little Horror thrown in. There are a wide variety of authors and genres that I enjoy, though I've tended to skip some of the mainstays in favor of the more obscure. The wonderful thing about books is that for every seemingly fresh or original idea seen in TV or movies, a book has often explored a deeper variation decades ago. The concepts are simply much more inventive in novels than you can ever conceptualize on the screen, though I do enjoy both.
Some of my favorite fantasy novels were the Rift War series by Raymond E. Feist, as well as the Chronicles of Prydain by Lloyd Alexander. I also liked the Wheel of Time series by Robert Jordan and the Sword of Truth series by Terry Goodkind, even though the two seemed very similar and tended to be heavily padded. The First Law is a very good, gritty new series by Joe Abercrombie. There are also some very good humorous fantasies. The Myth series by Robert Asprin was always pretty funny, and Terry Pratchett's Disc World is one of the best series I've ever read.
There are some very interesting alternate history stories, some of which consider rather large differences. Harry Harrison's Eden series canceled out the dinosaur extinction and was about a young human slave of intelligent dinosaurs who utilized bio-engineered technology. Harry Turtledove's A Different Flesh was about a North America inhabited by Hominids rather than Native Americans when the first settlers arrived. Written by Bruce Sterling and William Gibson, the two biggest names in Cyberpunk, The Difference Engine spanned Europe and the American West if the early version of the computer had been utilized. Orson Scott Card's Alvin Maker series was a really interesting tale of a 7th son of a 7th son in an American West of magic and hexes. Phillip Pullman's His Dark Materials, partially a response to the Chronicles of Narnia, is a truly grand series about alternate worlds tied together by misunderstood 'original sin'. In more modern times, Fatherland by Robert Harris is a very cool detective story taking place after Hitler's victory. Michael Chabon's The Yiddish Policemen's Union is about a Jewish state in Alaska rather than Israel, whose time is about to run out.
In a similar bent, there are also stories about time travel that often end up in alternate histories. Tim Power's Anubis Gates is a wild ride through time amongst the Romantic Poets. Julian May's Pliocene Exiles was an extremely imaginative series about people stranded millions of years in the past, enslaved by two rival groups of aliens who spawned our myths of Faeries. Conquistador by S.M. Stirling examined the results of a WWII vet discovering a portal to a past version of Earth and taking full advantage of it. G.O.D. Inc., written by one of my favorite authors Jack L. Chalker, was about a sinister syndicate of time travelers who ruled their own versions of Earth and would invade new versions such as our own by manipulating the local criminal elements.
Down the Bright Way by Robert Reed was an interesting story about alien worlds linked together by an ancient pathway. The characters included many bizarre variations of humanity spread across these various worlds. While mostly known for his horror, Clive Barker also wrote some interesting novels about other-worldly connections; Weave World and Imajica. I've always been impressed by how unique his monsters tend to be; frightening in a very unconventional way. However the king of these types of stories would be Roger Zelazny. Roadmarks and A Dark Traveling are both interesting journeys through different worlds. Probably his biggest series, the Amber Chronicles are about a young man who has to piece together his connection to the Court of Amber, a dimension that other worlds are merely shadows of. Similarly, the Changeling Saga is also about a young man who realizes his connection to another magical world. Zelazny also wrote intriguing stories about individuals with mysterious power and purpose, such as This Immortal, Lord of Light, Creatures of Light and Darkness, and Isle of the Dead.
Parke Godwin's Snake Oil Variations was an interesting series of adventures that occurred in the afterlife. To Reign in Hell by Steven Brust is a somewhat well-known take on Paradise Lost, further exploring a different angle to the Fall.