Le Morte d'Arthur: When most people think of the Arthurian myth, it is the version laid down here by a mercenary knight at the tail-end of the age of chivalry that they're thinking of. On the advice of the "Suggested Reading" section at the back of Pendragon I picked up the Penguin edition, which does a nice job of balancing readability with archaic diction. But I also have a "modernized" edition I can turn to if I just want to quickly reference a particular section to use in a game. Apart from Gawaine and the Green Knight, I have yet to read any other medieval versions of the myth like the Vulgate or the Mabinogion, something I hope to correct in the near future.
The Acts of King Arthur and His Noble Knights by John Steinbeck. T. H. White gets a lot of attention, and deservedly so, but his take on Arthur is just a bit too idiosyncratic for gaming inspiration purpopses. For a modern treatment of the mythology, this is my favorite collection.
The Arthurian Companion by Phyllis Ann Karr. Absolutely indispensable. An encyclopedia of people, places, and things exhaustively compiled from the full range of available medieval sources.
Castles by Alan Lee. In some ways this was where it all started. I received this book as a gift (birthday? Christmas?) back during the height of my childhood interest in things medieval (about age 11-12). I still reference Lee's gorgeously evocative watercolors (example at left) for inspiration, and the well-written "thumbnail myths" that accompany them (scribed by David Day) have provided both direct and indirect material for some of my past scenarios.
Medieval Knights by David Nicolle. This is a representative entry for children's books in general. Yes, kid's books, the greatest friend of the harried GM ever invented. Particularly books on history, mythology, and other fun subjects. The big quarto or folio-sized, full-color jobbies with lots of pretty pictures are the way to go: like the veritable mother bird preparing to feed her nestlings, flip through their pages and gobble up the visual inspiration for later regurgitation at the game table.
Life in a Medieval Castle and Village Coloring Book by John Green. Another representative entry. Forget Osprey - there are tons of great, cheap Dover-style paperbacks available with original line art depicting arms, armor, castles, and scenes of daily life. My most recent addition to this list; I wish I'd had these sorts of books from the get-go.
A Distant Mirror by Barbara Tuchman. The 14th-century was arguably the Middle Ages at their worst, and Tuchman paints a brilliant picture of church corruption, mercenary knights, plague, excess, war, famine, and peasant revolts. Indispensable for depicting the darker side of chivalry and the Arthurian cycle.
Excalibur (1981). Movies were my real introduction to the wonder of Arthur and the Middle Ages. I saw a TV-edited version of Excalibur when I was about 9 years old and it rocked my world. This movie is still my touchstone for what an epic Pendragon campaign should feel like, and Nicol Williamson's Merlin will never be bettered in my opinion.
"Come father, let us embrace at last!"
The Lion in Winter (1968). Great sets, great costumes (especially for the time it was made), absolutely indispensable for court politics and the true heart of Pendragon: the rack of human relationships.
"Poor John. Who says poor John? Don't everybody sob at once! My God, if I went up in flames there's not a living soul who'd pee on me to put the fire out!" "Let's strike a flint and see."
Knightriders (1981). An under-appreciated entry in George Romero's oeuvre, despite its modern setting this movie is the best treatment I've yet seen of the clash between chivalric ideals and hard reality. Plus, if you want a treatment of Merlin-as-hippy as an alternative to Nicol Williamson's take, look no further. (Oh, and Des thinks Tom Savini is sex on toast in this film, for what it's worth.)
"No, it's just getting too tough. It's tough to live by the code. I mean, it's real hard to live for something that you believe in. People try it and then they get tired of it, like they get tired of their...diets. Or exercise. Or their marriage. Or their kids, or their job, or themselves...or they get tired of their God."
Monty Python and the Holy Grail (1975). The most obscure entry on the list, apparently this was a comedy movie released by some nobody British troupe back in the 70s. Mainly useful for its fantastic attention to detail in sets and costumes and the following clip, which is an apt summation of what happens when every knight in your party fails their Valorous rolls.
"We'll not risk another frontal assault - that rabbit's dynamite!"
Bonus Section: Ludography
As per a regular reader's request, here is my personal list of Pendragon supplements I've found most useful, given more or less in order of precedence. All are available as PDF and/or POD selections from DriveThruRPG.
The Great Pendragon Campaign: Absolutely indispensable, even if you don't plan on running the whole hog. Jam packed with scenarios, regional detail, NPCs (although, unfortunately, not their stats; see the GM Characters PDF for that), and a background narrative that keeps you up to date on what's going down with the wider world.
Greg Stafford's Pendragon Page: Not a book per se, but just as useful, Greg Stafford's personal Pendragon page is a wonderful cornucopia of essays on the setting, new rules, house rules, regional details, and lots more. The biggest problem with this page is deciding what to include in your campaign and what to leave out!
The Book of Knights and Ladies: The current (fifth) edition assumes you're creating knights from Salisbury in the year 485 during the reign of Uther. This book opens character creation up to knights from any part of Britain (and beyond) in any period of the saga.
Tales of Mystic Tournaments: All the "Tales of..." collections are worth a look (particularly Spectre Kings) but this is my favorite. As the name implies, it collects three adventures centering around tournaments, plus it includes rules for feasting and events therein (always a favorite part of my group's Pendragon experience). This is the collection that includes "The Grey Knight" and "The Tournament of Dreams", either one an excellent way to kick off a campaign (although I'd bump the latter up to the 530s at the earliest in order to fit in better with the chronology).
Blood and Lust: The best of the early-90s regional sourcebooks thanks to the inclusion of "The Heart Blade" story arc, my all-time favorite Pendragon adventure.
The Book of Knights: Greg Stafford is understandably cool towards the Green Knight-era Pendragon publications, but I have to say that this is my favorite treatment of the core rules: excellent layout, editing, and presentation provide an ideal introduction to Pendragon novices (GM and player alike).
Savage Mountains: As much as Blood and Lust is a sentimental favorite, I'll admit it's a bit uneven as a cohesive sourcebook. This sourcebook on Cambria (i.e. Wales) presents an integrated collection of information and (outstanding) scenarios that would allow you to run an entire campaign set among the eponymous mountains. If you're running the Great Pendragon Campaign, be aware that some of the scenarios in here have been placed into the GPC chronology at specific points.
Beyond the Wall: Does for Scotland what Savage Mountains did for Wales. The scenarios aren't quite as good, but there's lots of truly outstanding information on the kingdoms and Pictish tribes of the region to make up for that. Plus the simplified Battle system in the Appendix has become my go-to system for handling mass combat.
Not exactly essential, I've nonetheless gotten enough out of the following titles to feel I'd be remiss in not including them.
Pendragon 4th Edition: You've got the 5th edition, why would you need the older version? Mainly because it gives you options to expand beyond the narrow scope presented in the current version of the game. If you have a player who really wants to play a sorcerer or enchantress, the rules are here. Details on Salisbury during the height of Arthur's reign are worth the price of admission alone if you're running the default campaign.
Lordly Domains: Until the Book of the Manor is available in PDF, this is the go-to source for rules on domain management (albeit at a rather more complex level that the BotM). It also includes a much more detailed system for hunting and falconry that's a lot of fun to play around with if your players are particularly interested in that side of play.
Perilous Forest: Most of the material in here dealing with the Wastelands is repeated or expanded on in the GPC, but this is still a useful reference if you plan on running a campaign set in the North and features a couple well-written scenarios that will likely be making an appearance in my own Solo GPC campaign.