I’m probably late to this party, but I’ve discovered that you can find incredible deals on used CS textbooks at Amazon, especially for older editions.
For example, I recently ordered a copy of Programming Language Pragmatics, by Michael L. Scott. It’s $63 new for the hardcover or $43 on a Kindle. I got a used copy of the (somewhat older) second edition for $3 + postage, for a total of $7. True, I don’t get the new chapter on VMs, but I can live with that. The third edition totally dried up the market for the second, so there are really good deals available!
The book itself is good so far, though I haven’t read enough to give a full review. It covers substantially similar territory as the dragon book, except more scope talking about programming languages and computer architecture and less depth talking about compiler internals. They’re both worth reading.