Zilog Z-80 User's Manual:
http://www.zilog.com/appnotes_download.php?FromPage=DirectLi...
Design For A Brain - W. Ross Ashby
https://archive.org/download/designforbrain00ashb/designforb...
Information Theory, Inference, and Learning Algorithms - David MacKay
http://www.inference.phy.cam.ac.uk/itprnn/book.pdf
Experiments with Alternate Currents of High Potential and High Frequency - Nikola Tesla
http://www.gutenberg.org/ebooks/13476.epub.images
Capitalism - A Treatise on Economics
http://www.capitalism.net/Capitalism/CAPITALISM_Internet.pdf
AMD64 Programmer's Manual (Volume 1)
http://support.amd.com/TechDocs/24592.pdf
AMD64 Programmer's Manual (Volume 2)
http://developer.amd.com/wordpress/media/2012/10/24593_APM_v...
AMD64 Programmer's Manual (Volume 3)
https://support.amd.com/TechDocs/24594.pdf
If you like sci-fi, check out the Baen Free Library:
http://www.baen.com/categories/free-library.html
and of course there's bound to be something at Project Gutenberg that you'd find interesting. You might find it interesting to go browse around there a bit.
Also note that you can export Wikipedia articles as PDF and you can even compose "books" made up of multiple Wikipedia articles. So if there's some topic you've been meaning to read up on, maybe go make yourself a WP book on that subject and take that along on your trip.