Reddit reviews The Monsters and the Critics: And Other Essays. J.R.R. Tolkien

Sure. Stop me when this gets boring!

The History of Middle-earth.

The History of the Hobbit.

The Road to Middle-earth, J.R.R. Tolkien: Author of the Century and Roots and Branches, all by Tom Shippey

You should read Tolkien's Letters, too.

Other books to consider:

The LOTR reader's Companion

J.R.R. Tolkien and His Literary Resonances

Perilous Realms: Celtic and Norse in Tolkien's Middle-earth

The Keys of Middle-Earth: Discovering Medieval Literature through the Fiction of J.R.R. Tolkien

Understanding The Lord of the Rings: The Best of Tolkien Criticism

J.R.R. Tolkien Companion and Guide

If you're feeling rich, you could try to find a copy of Songs For The Philologists, a collection of poems, mostly in Old English, written by Tolkien and E.V. Gordon (I only have a .pdf copy).

I'd also read Tolkien's Beowulf criticism.

and just for fun, read Ents, Elves, and Eriador: The Environmental Vision of J.R.R. Tolkien, which is nothing to do with philology but which was cowritten by my major professor :)

Let's see, what else? Anything by Douglas A. Anderson, Verlyn Flieger or Michael Drout (especially Drout's Beowulf and the Critics and How Tradition Works: A Meme-Based Cultural Poetics of the Anglo-Saxon Tenth Century.

That's pretty much all that leaps immediately to mind, just glancing over my bookshelves, but if you search for "Tolkien scholarship and criticism" you will find much, much more. Hope this helps!

I'd probably start here in order to get an idea of Britain on the macro scale.
While bountyonme mentions Saxons ruling from 410AD to 1066, the term "rule" is rather loose, as they were constantly being attacked and invaded by Nordic peoples, even having one as their king for a period of time.
And before even that, there was the Roman rule of England, which shouldn't be overlooked, as the Romans had a major impact on the entirety of Europe, and with England so far away from Roman Empire's Mediterranean hub, there was a different flavor of Roman imperialism there than in Italy or France, which left its mark on Britain even after the fall of Rome
Finally, when you're looking at British history, if you're curious about the various cultural elements of the Isles, and how the Brits perceived themselves and others, I would suggest reading Beowulf, with Tolkien's essay on it which examines how the tale provides a peek into the cultural mindset of the period. Also, if you want a somewhat amusing read, check out Gerald of Wales' Topography of Ireland, the section on Irish miracles is especially interesting.
Also, returning to bountyonme, definitely definitely definitely read The Canterbury Tales, Chaucer's poetry is equal parts beautiful and humorous. I would suggest reading it in its original Middle English, if you can get a handle on it. Many of the translations remove much of the beauty and humor of the work, leaving a dry, lifeless piece of shit and besmirching the genius's name. I would suggest this edition if you can handle the price or get it through interlibrary loan, it's well-footnoted, provides a primer on how to read/understand Middle English, and uses the most complete edition of the texts.
Welcome to the confusing/frustrating/entertaining/well-worthwhile arena of Medieval history.

If you have not already done so, I would strongly urge you to read A Secret Vice, the book not just the essay. The book was published only a few years ago, I think. It serves as an insight into Professor Tolkien’s views on languages, both real and invented.

I would also recommend reading the Professor’s essay English and Welsh, published in the book The Monsters and the Critics and Other Essays.

Another recommendation is David Salo’s A Gateway to Sindarin, though it is primarily a grammar of his Neo-Sindarin for the LOTR movies, he does explore Tolkien’s methods for inventing languages and his inspirations.

If you're looking for a good translation of Beowulf, I'm a huge personal fan of Howell Chickering's translation (most editions have are dual-language, so you can see the OE verse on the left and Chickering's ME verse on the right)! He does a fantastic job of integrating the original verse form of the lines while maintaining a pretty exact meaning.

It may also be worthwhile investigating this collection

http://www.amazon.com/Monsters-Critics-J-R-Tolkien/dp/026110263X/ref=sr_1_28?s=books&ie=UTF8&qid=1322377425&sr=1-28