John Garth's Tolkien and the Great War was just released as an ebook in the USA in November, so for those of you who have been waiting to read this excellent book, this is a great opportunity.
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Tolkien and the Great War: The Threshold of Middle-earth

Houghton Mifflin Harcourt (2013-06-11)


$2.99 Amazon.com (Kindle Edition) - Availability: Now


Shippey's Road to Middle-earth is a classic.
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The Road to Middle-Earth: How J. R. R. Tolkien Created a New Mythology

Mariner Books (2014-04-08)


$2.99 Amazon.com (Kindle Edition) - Availability: Now


Not sure if this one is on sale today, but a great price nonetheless (it is the complete one-volume edition):
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The History of the Hobbit: Mr Baggins and Return to Bag-End


£4.99 Amazon.co.uk (Kindle Edition) - Availability: Now
€2.77 Amazon.de (Kindle Ausgabe) - Availability: Now