From the article at http://coloringbookaddict.com/top-10-stress-relieving-adult-coloring-books/ .  

See http://coloringbookaddict.com/coloring-books-science-fiction-geeks for a good list of geek-y coloring books.  At bottom of the article there are also links to download free pages, including but not limited Tolkien, Harry Potter and Doctor Who.

Not pictured above:

 Harry Potter: The Official Coloring Book #1   

 The Walking Dead Coloring Book - Robert Kirkman

 Doctor Who Travels in Time Coloring Book   

Tolkien's World: A Fantasy Coloring Book - Mauro Mazzara  

The Official A Game of Thrones Coloring Book - George R. R. Martin

Star Wars: 100 Images to Inspire Creativity and Relaxation - Catherine Saunier-Talec

  ;

Supernatural: The Official Coloring Book - Insight Editions  

Star Trek: The Original Series Adult Coloring Book - CBS  

 The Official Outlander Coloring Book - Diana Gabaldon  

Buffy the Vampire Slayer Adult Coloring Book - Fox

Steampunk Coloring Book 2 (Volume 2) - Nick Snels  

The Hound of the Baskervilles - John Green, Arthur Conan Doyle 

Yes, I know.  I totally missed August 2, National Coloring Day.  But, the book is still available for free download at http://www.doverpublications.com/coloringbookday/ .

See http://www.doverpublications.com/zb/samples/799026/sample8e.html for full page images of what I'm showing only a brief glimpse of (for copyright reasons):

Doesn't require account, signup, joining mailing list or anything.  Just a few freebies.  Even answer page for hidden images is free.  Just some older ones I remembered now that adult coloring books are trending.

This just cracked me up for some reason (ran across from Lobstergirl on goodreads popcorn thread about allowing coloring books if adult but not comic book single issues).