I've found The Little Schemer[1] to be very approachable and effective for grokking common CS concepts.
[1]: http://www.amazon.com/The-Little-Schemer-4th-Edition/dp/0262...
[1]: http://www.amazon.com/The-Little-Schemer-4th-Edition/dp/0262...
Most exercises don't require an interpreter, which is nice if you need a break from screentime. But I do enjoy working through some of them in Racket or even Pixie Scheme. TLS isn't free or available in a digital format (AFAIK), but the book is inexpensive and very portable.
Also, I want to point out that "essential readings" is not the right attitude. In order to really make progress you're going to need to get your hands dirty and work though some examples. One very good exercise is to write an interpreter for a language you invent.