Apress‘ ebook deal of the day is “Practical Common Lisp” for just $10. Now I have a PDF stack growing next to the dead tree one.
Practical Common Lisp
Published
Fighting the automation paradox || Deployment θα κάνουμε φωνάζοντας "αέρα"
Managing organized complexity 
I have a dead-tree edition of PCL, right next to my bed.
It has served me quite well since March 2006, when I bought it from Border’s. The PDF version seems to cost about 1/6 of the original price of the printed hardcover version. That’s a very good price :-)
Well, the normal eBook price is $41.99. But Apress sells a different book title for $10 every day. I check their “deal of the day” from time to time and bingo! 1/6th of the price! Plus, since PCL has served you well, I am damn sure it will do the same for me :)