You will find a "boat load" of free ebooks at http://www.monergism.com/ just by searching. The site offers epub and mobi format books but the great thing is that you can take the epub file, use the free calibre e-book tool (http://calibre-ebook.com/) and convert the epub to the rtf format. Then open the rtf in Word, save the book cover as a jpg file, then save the rtf as a docx file and now you have the makings for a new personal book for your Logos collection.
So while this post does not include a file it does provide information on where to find many :-)
Blessings