These authors sought the mindsprings of Middle-Earth in literature, locations, and historical events or documents.  There is a medieval studies section at the bottom of the page.
Mythology & Sources

The Mythology of Middle-Earth

Houghton Mifflin, Ruth S. Noel

1977 Hardcover   ISBN : 0395250064

1977 Softcover   ISBN : 0395272084

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0395250064
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0395272084

One Ring to Bind Them All :
Tolkien's Mythologies

University of Alabama Press, Anne C. Petty

1979 Hardcover   ISBN : 0817373284

1984 Softcover   ISBN : 0817302158

2002 Softcover   ISBN : 0817312056

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0817373284
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0817302158
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0817312056

England and Always

Eerdmans, Jared Lobdell

1981 Softcover   ISBN : 0802818986

UMI Print-on-Demand

Hardcover   ISBN : 0802818986

The World of the Rings :
Language, Religion, and Adventure in Tolkien

Revised edition

2004 Softcover   ISBN : 0812695690

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0802818986
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0812695690

Tolkien's Ring

David Day, Illustrated by Alan Lee

Barnes & Noble

2002 Hardcover   ISBN : 1586635271

Pavilion; Reprint edition

2012 Hardcover   ISBN : 1862059276

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1586635271
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1862059276

War and the Works of J.R.R. Tolkien

Praeger Publishers, Janet Brennan Croft

2004 Hardcover   ISBN : 0313325928

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Tolkien's Mythology for England

Anglo-Saxon Books, Edmund Wainwright

2004 Hardcover   ISBN : 189828136X

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The World of Tolkien :
Mythological Sources of The Lord of the Rings

Grammercy; David Day

2004 Hardcover   ISBN : 0517223171

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There and Back Again :
In the Footsteps of J.R.R. Tolkien's England

Cadogan Guides,  Mathew Lyons

2004 Hardcover   ISBN : 1860111394

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Perilous Realms :
Celtic And Norse in Tolkien's Middle-Earth

University of Toronto Press, Marjorie Burns

2005 Hardcover   ISBN : 0802038719

2005 Softcover   ISBN : 0802038069

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0802038719
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0802038069

Evolution Of Tolkien's Mythology :
A Study of the History of Middle-earth (Critical Explorations in Science Fiction and Fantasy)

McFarland, Elizabeth A. Whittingham (Editor)

2007 Softcover    ISBN-10 : 0786432810
ISBN-13 : 9780786432813

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The History of the Hobbit

Houghton Mifflin, Hardcover 2007, John D. Rateliff

Boxed with The Hobbit   ISBN-10 : 0618964401
ISBN-13 : 9780618964406

Vol.1: Mr. Baggins    ISBN-10 : 0618968474
ISBN-13 : 9780618968473

Vol.2: Return to Bag End    ISBN-10 : 0618969195
ISBN-13 : 9780618969197

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0618964401
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0618968474
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0618969195
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9780007440825

Quest for Middle-Earth

iUniverse-Indigo,   Dirk Vander Ploeg

2007 Softcover   ISBN : 0595440932   9780595440931

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Medieval Studies

Tolkien the Medievalist

Routledge,   Jane Chance (Editor)

2003 Hardcover   ISBN : 0415289440

2008 Softcover   ISBN : 0415473489
   ISBN-13 : 9780415473484

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Tolkien in the Land of Heroes :
Discovering the Human Spirit

Cold Spring Press, Anne C. Petty (Author), J. Stein (Editor)

2003 Softcover   ISBN : 1892975998

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Myth & Middle-Earth :
Exploring the Medieval Legends Behind J.R.R. Tolkien's The Hobbit & The Lord of the Rings

Cold Spring Press, Leslie Ellen Jones

2002 Softcover   ISBN : 1892975815

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Tolkien's Modern Middle Ages

Palgrave Macmillan, Jane Chance & Alfred Siewers (Editors)

2005 Hardcover   ISBN : 1403969736

2009 Softcover   ISBN : 0230616798
   ISBN-13 : 9780230616790

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

Palgrave Macmillan, Stuart Lee & Elizabeth Solopova

2006 Paperback   ISBN : 140394671X

2006 Hardcover   ISBN : 1403946728

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140394671X
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1403946728