The Pillars of the Earth by Ken Follett
New York: William Morrow and Company, Inc., 1989.

This is an excellent book that I highly recommend.  It follows several characters and their intertwined lives over the second and third quarters of the twelfth century.  Becket appears at the end, but only for his martyrdom.  Except for a couple of very minor things, the historical accuracy is commendable.  And I like the author's last name, from the same derivation as Foliot - little fool.
 

When Christ and His Saints Slept by Sharon Kay Penman
New York: Henry Holt and Company, 1995.

The story of the troubles between Stephen and Matilda.  Almost all of it is documented history told as a novel.  The young Becket makes a cameo for a page or two.

Since I first wrote this, the sequel has come out, Time and Chance, dealing with Henry, Eleanor, and Thomas.  I haven't read it, so I can't really offer an opinion, but I did look through it in the bookstore in search of the martyrdom (direness, familiar to my slaughterous thoughts, cannot once start me), and I have to say that Follett does a better job of making you care.

The Catherine LeVendeur Mysteries by Sharan Newman

A whole series of mysteries set in the twelfth century, featuring Catherine and her family.  I like that the fact that the books don't focus mainly on the mystery aspects but incorporate personal relationships, family happenings, and so on.
 

The Wearing of Costume by Ruth M. Green
London: Pittman, 1966, reprinted New York: Drama Publishers, 1995.

If you need to know how to move in period dress, or if you just need to know how to manage a long skirt, this is the book to consult.  It covers both men's and women's clothes through the centuries, including standards of modesty for each time period, how clothing affected posture and movements, and how to handle clothes so that you not only don't get tangled but look as though this is your accustomed apparel.

This book was responsible for making me notice one of the minor things that bothered me in The Pillars of the Earth: Aliena crossing her legs.  That's a modern thing.  For women wearing floor-length dresses it was actually appropriate to sit with their legs apart.  Even now, crossing one's legs isn't exactly refined, which is why they teach you to cross your ankles in dancing class.

Making Saints by Kenneth L. Woodward
New York: Simon and Schuster, 1990.

The (modern) process of canonization and what qualifications are needed to become saints.