Code by Charles Petzold will feel a bit slow at the beginning but gets pretty fun once he gets to circuits and builds up a whole functioning computer.
Hackers by Steven Levy is a great history of the 50s to 80s. Very focused on the personalities of the non-establishment people who made things happen.
Go To by Steve Lohr is also a history and super boring. Give it a pass.
The Mythical Man-Month by Frederick Brooks--if you haven't read this yet, it's pretty light, and it's amazing how contemporary it feels.
Programmers at Work by Susan Lammers: interviews of lots of famous computer pioneers. Very interesting to think about the remarks that appear again and again.
Smart and Gets Things Done by Joel Spolsky will make you feel good about yourself. :-)
The Mac is Not a Typewriter by Robin Williams is a nice bite-sized lesson on type.
The original Human Interface Guidelines by Apple is a pretty interesting look back into what motivated them in those days, especially if you do any iOS development.
The Visual Display of Quantitative Information by Tufte is not something you can carry on a bus but a pretty fun read.
Ghost in the Wires by Kevin Mitnick is like a novel. I'm not sure I believe it all, but it's interesting.
The Elements of Style by Strunk & White is tiny and will improve your writing a lot.
Clear and Simple as the Truth by Thomas & Turner is good after you've digested Strunk & White a bit.
Time Management for System Administrators by Limoncelli might give you some things to think about.
Expert C Programming: Deep C Secrets by Peter van der Linden is technical but super amusing and helpful if you've done any C before.
Programming Collective Intelligence by Toby Segaran is a light overview of various machine learning techniques. Enough to make you sound like you know what you're talking about. :-)
Release It! by Michael Nygard and Scalable Internet Architectures by Theo Schlossnagle have good lessons on building reliable systems. The latter is more technical, even offering code for some things; the former, more high-level.
Effective Java by Bloch has good tips if you do any Java. After 10+ years I still remember his idea that inheritance is a "scarce resource" and should be used sparingly. The discussion of `equals()` might convince you to switch to Ruby. :-)
How about math? Get any collection of essays by Martin Gardner. The Colossal Book of Mathematics is more a bedside/restroom book than one for a bus, but they also publish many thin collections like Hexaflexagons, Probability Paradoxes, and the Tower of Hanoi.
Javascript: The Good Parts by Crockford is tiny and a classic.
