I've often wanted to review the A Song of Ice and Fire books on this website but, really, there's nothing I could possibly say about them which other websites have not said and better. Likewise, there's no point in bringing more attention to the novels because they're already some of the most famous in fantasy. It's pretty much the same with the spin-offs as there's not much point in talking about Game of Thrones since everyone and their brother is watching that show.
I've made an exception for Telltale's Game of Thrones because that's a side-story but this is the first book I feel comfortable recommending which might have slipped under fans' radars. A Knight of the Seven Kingdoms is a compilation of three novellas (The Hedge Knight, The Sworn Sword, and The Mystery Knight) written by George R.R. Martin for various anthologies.
The premise is a young hedge knight, Ser Duncan the Tall (a self-granted title since his master never knighted him), takes on a squire named Egg after the death of his mentor Ser Arlan. A hedge knight is a knight with no prestige or lineage but has the training as well as equipment to be a mounted soldier in the Seven Kingdoms. At the absolute bottom of the social hierarchy's warrior class but still part of it, Duncan has a unique perspective on events occurring in the century before A Game of Thrones.
The three novellas take place in a very different Westeros from the one described in the books because the Targaryens are still at, if not the height of the rule then some distance from their twilight. The lands have been at peace for a decade and the nobility, if not following the example of chivalry in spirit, is at least trying to follow the example of chivalry in appearance. In a very real way, the book serves as an argument for a Targaryen Restoration because it shows everyone more or less getting along and the peasants able to live reasonably secure lives.
George R.R. Martin, the father of grimdark, still treats the Medieval romance with a good deal of disdain but it's not nearly as cynical in many respects. Ser Duncan's basic decency makes him a far better knight than those born into the role but the absence of Gregor Cleganes, Boltons, Bloody Mummers, and even Lannisters make the villains of a decidedly more sympathetic bent. They're still very realistic fantasy with only the occasional prophetic dream keeping it from being absent magic together but the heart of the stories is a peasant-born warrior trying to navigate the complicated social dynamics of Westeros' knightly class.
The Hedge Knight is, in a weird way, not that dissimilar from Heath Ledger's A Knight's Tale. Duncan is a peasant knight from Fleabottom who has a vision of becoming a famous warrior after Ser Arlan's death. Unfortunately, Ser Duncan lacks William Thatcher's godlike skill with a lance and swiftly finds himself in hot water with a Targaryen prince. In a very real way, this is a sports story and the deadly stakes of the event make it all the more entertaining to read about. Of the three, The Hedge Knight is my least favorite as I never really found that much interest in jousting and its central role in peacetime Medieval life.
The Sworn Sword is a follow up to The Hedge Knight where Duncan has managed to find himself as an actual proper sworn knight to a lord--sort of. Having taken up service to a lord of something which barely qualifies as a tower, Duncan ends up caught up in a conflict between his lord and the beautiful widow across the river. The central conflict turns out to be not one of good and evil but the legacy of a war which had, to quote George Lucas, heroes on both sides. I like how it managed to take a very Medieval concept of fighting for a ladies' honor and play it straight while also illustrating how absurd it was.
The Mystery Knight is, bluntly, one of my favorite stories in fantasy. I've re-read this novella five times and am probably going to do so again. It's a story with a lot of parallels to Bonnie Prince Charlie's revolt and is basically a Medieval spy novel set against the backdrop of a tournament. I love the characters of Lord Butterwell, the Fiddler, and Fireball's bastard. They are eccentric, larger than life, and yet believable. I also loved finally getting a chance to meet
These stories are,obviously, going to be enjoyed more by fans of the books than by the show. Aside from the possible relationship between Duncan and Brienne, many of the details of the history will fly by television viewers. Despite this, I think they would be enjoyable even to those who have no experience with the world. Fans of grimdark will find the stories a good deal more idealistic and pleasant but still possessed of the moral ambiguity as well as "realism" which made the original books so enjoyable.
The chief draw of the books for me is the relationship between Ser Duncan and Egg. Duncan is, to be honest, dumb but decent while Egg is highly-intelligent and somewhat more ruthless than his master. The contrast between their social positions, viewpoints, and attitudes provides an endless array of interesting conversations. It's kind of sad I know how their story works out due to The World of Ice and Fire but their tale is one I could follow through its own series. They're hilarious, insightful, and fun together--and what more can you ask from your heroes?
In conclusion, I really really recommend this book. I almost wish George R.R. Martin would take more time from The Winds of Winter to do more of these stories. They're fun, light, and entertaining reads which deserve to be looked into. This version of the story is illustrated and while I tend to prefer the comic book versions of these story, it lends a sort of "Illustrated King Arthur" feel to things.