I'll divide my recommendations up:
Ellis Peters: the Cadfael
Twee murder mysteries in mid-12th century England.
Ken Follett: The Pillars of the Earth
The lives and times of people trying to build a cathedral in mid-12th century England. Part soap-opera, part architectural treatise, all "good read".
George Shipway: Knight in Anarchy
Brutal account of one squire's baptism of fire into knighthood at the time of the 12th century Anarchy. Excellent on combat, arms and armour descriptions -- but not for those with a weak stomach.
Katherine Kerr: The Deverry
Excellent fantasy series that builds from low magic beginnings to very high fantasy indeed.
JRR Tolkein: The Lord of the Rings
Nuff said, really.
Robert Bartlett: England under the Norman and Angevin Kings
If you only get one history book, this is the one to have. It covers just about everything. Absolutely superb -- I never travel anywhere without it.
Geis & Geis: Life in a Medieval Village
Flawed, but very readable.
DM Stenton: English Society in the Early Middle Ages
Inexpensive, novel-sized paperback. Very readable and informative -- perfect for reading on the train.
Ronald B Tobias: Twenty Master Plots and How to Build Them
Eventually we all run out of ideas. This is possibly the most RPG-relevant of several creative writing books I have on the bookshelf.
Films and TV
Robin of Sherwood
: superb British-made TV series from the early-80s. Atmospheric, moody, fake Celtic mysticism. I always think of Robin of Sherwood when I imagine Harn. And it's now available on DVD.
The little-known Patrick Bergin movie, made at the same time as Costner's abomination, and far, far better.
Its history sucked, but it's a good story -- and it's surprisingly good on a lot of the little details of clothing, weapons, animals and the like.
The 13th Warrior
Surprisingly good Vikings vs neanderthals romp. It's worth reading Eaters of the Dead
, the Crichton book it was based on, as well.
And, of course...
The Fellowship of the Ring
and The Two Towers