This is one of our most frequently asked questions. Usually we just wing it based on what we have on stock, but it seemed like it was time to make a list of our go-to books.
In all honestly, there really is nothing exactly like Martin's Song of Ice and Fire series. (It's also called the Game of Thrones series after the first book and the HBO series, which airs under that name). He is the premier writer of dark, realistic fantasy. So don't expect any of these to be the same as his books. But they are all well-written fantasy series for adult readers
Neither I nor my coworkers have read everything on here, but I've read enough first chapters to assure myself that the writing is good, and pruned through enough reviews to have a reasonably good sense that they are all well-liked by most readers (4 to 5 stars on Amazon, and good reviews from my customers). That's why there are a couple of big-name authors whose books you will not find on this list. If there are 10+ books in a series but they start to stink after book six...who needs that? There's too much other fabulous stuff out there.
In alphabetical order by author's name.
Joe Abercrombie. First Law trilogy: The Blade Itself; Before They Are Hanged; Last Argument Of Kings
R. Scott Bakker. Prince of Nothing trilogy: The Darkness That Comes Before; The Warrior-Prophet; The Thousandfold Thought.
Anne Bishop. Black Jewels series: Daughter of the Blood; Heir to the Shadows; Queen of the Darkness; The Invisible Ring; Dreams Made Flesh; Tangled Webs; The Shadow Queen; Shalador's Lady; Twilight's Dawn.
Lois McMaster Bujold. Curse of Chalion series: The Curse of Chalion, Paladin of Souls, The Hallowed Hunt.
Lois McMaster Bujold. Sharing Knife series: Beguilement; Legacy; Passage; Horizon.
Jim Butcher. Codex Alera series: Furies of Calderon; Academ's Fury; Cursor's Fury; Captain's Fury; Princeps' Fury; First Lord's Fury.
Jacqueline Carey. Kushiel's Legacy series: Kushiel's Dart; Kushiel's Chosen; Kushiel's Avatar; Kushiel's Scion; Kushiel's Justice; Kushiel's Mercy; Naamah's Kiss; Naamah's Curse; Naamah's Blessing.
Glenn Cook. Black Company series: The Black Company; Shadows Linger; The White Rose; The Silver Spike.
Diane Duane. Tales of the Five: The Door into Fire; The Door into Shadow; The Door into Sunset.
David Anthony Durham. Acacia Trilogy: Acacia; The Other Lands; The Sacred Band.
Steven Erikson. Malazan Book of the Fallen series: Gardens of the Moon; Deadhouse Gates; Memories of Ice; House of Chains; Midnight Tides; The Bonehunters; Reaper's Gale; Toll the Hounds; Dust of Dreams; The Crippled God.
C. S. Friedman. Coldfire trilogy: Black Sun Rising; When True Night Falls; Crown of Shadows.
C. S. Friedman. Magister trilogy: Feast of Souls; The Wings of Wrath; Legacy of Kings.
Barbara Hambly. Darwath series. The Time of the Dark; The Walls of Air; The Armies of Daylight; Mother of Winter; Icefalcon's Quest
Barbara Hambly. Unschooled Wizard series. The Ladies of Mandrigyn; The Witches of Wenshar; The Dark Hand of Magic.
Robin Hobb. Farseer trilogy: Assassin's Apprentice; Royal Assassin; Assassin's Quest.
Robin Hobb. Liveship Traders trilogy: Ship of Magic; The Mad Ship; Ship of Destiny.
Robin Hobb. Tawny Man trilogy: Fool's Errand; The Golden Fool; Fool's Fate.
N. K. Jemisin. Inheritance trilogy: The Hundred Thousand Kingdoms; The Broken Kingdoms; The Kingdom of Gods.
Guy Gavriel Kay. Two stand-alones: The Lions of Al-Rassan and Tigana.
Stephen King. The Dark Tower series: The Gunslinger; The Drawing of the Three; The Waste Lands; Wizard and Glass; Wolves of the Calla; Song of Susannah; The Dark Tower; The Wind Through the Keyhole.
Scott Lynch. Gentleman Bastard series. The Lies of Locke Lamora; Red Seas Under Red Skies; The Republic of Thieves.
Naomi Novik. Temeraire series: Temeraire (aka His Majesty's Dragon); The Throne of Jade; Black Powder War; Empire of Ivory; Victory of Eagles; Tongues of Serpents; Crucible of Gold; Blood of Tyrants.
Patrick Rothfuss. Kingkiller Chronicles. The Name of the Wind; The Wise Man's Fear.
Brandon Sanderson. Mistborn series: The Final Empire; The Well of Ascension; The Hero of Ages; The Alloy of Law.
Matthew Stover. Overworld series: Heroes Die; Blade of Tyshall.
Judith Tarr. Avaryan Rising series: The Hall of the Mountain King; The Lady of Han-Gilen; A Fall of Princes; Arrows of the Sun; Spear of Heaven; Tides of Darkness.
J. R. R. Tolkien. Lord of the Rings trilogy. The Fellowship of the Ring; The Two Towers; The Return of the King.
Brent Weeks. Night Angel trilogy: The Way of Shadows; Shadow's Edge; Beyond the Shadows.
Tad Williams. Memory, Sorrow and Thorn trilogy: The Dragonbone Chair; The Stone of Farewell; To Green Angel Tower (which is published in two volumes as Part 1 and Part 2).
Gene Wolfe. Urth: Book of the New Sun series. The Shadow of the Torturer; The Claw of the Conciliator; The Sword of the Lictor; The Citadel of the Autarch; The Urth of the New Sun.
A nice reference describing some of these works is io9's article 10 Great Fantasy Series to Read While You’re Waiting for George R.R. Martin’s Next Book.