Are you sure that Shankle's book is in the public domain? I'm unable to find any biographical information about him, but he appears to have been releasing books as recently as 1955, meaning that his work is not in the public domain in either the United States or any country with a "life + 70 years" copyright term. If the work is protected by copyright, you can't legally release your book, no matter how much work you've put into it. It's a "derivative work", and would require the permission of the original copyright holder to publish.
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