I found The Cuckoo's Egg second-hand for 50c. Easily the greatest enjoyment:price ratio of anything I own. Exploding the Phone is also phreaking awesome. I devoured that book over the space of a few hours.
Yeah, The cuckoo's egg would be the greatest price:enjoyment ratio, but then I bought Half-Life, Blud Shift, Opposing Force, Day of Defeat, Counterstrike, Riccochet, and TF Classic for $5. And I got the cuckoo's egg for $7...