Zilog Z-80 User's Manual:
Design For A Brain - W. Ross Ashby
Information Theory, Inference, and Learning Algorithms - David MacKay
Experiments with Alternate Currents of High Potential and High Frequency - Nikola Tesla
Capitalism - A Treatise on Economics
AMD64 Programmer's Manual (Volume 1)
AMD64 Programmer's Manual (Volume 2)
AMD64 Programmer's Manual (Volume 3)
If you like sci-fi, check out the Baen Free Library:
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.
In terms of gaining a solid understanding of the math and theory underlying most of modern machine learning/AI, the best resource is definitely the book "Information theory, inference and learning algorithms" by David MacKay.
The author is such a stand-up guy, he's made it available for free: http://www.inference.phy.cam.ac.uk/itprnn/book.pdf