A true first edition of Dr. Seuss' "The Cat in the Hat" (with dust jacket) is worth thousands. I found one in my parent's attic!
There are websites and chat boards devoted to the art of dumpster diving, the hows, wheres, legalities, finds, etc.
There is also at least one interesting blog operated by a girl who goes around photographing the trash in her neighborhood each week (what people are throwing out), and documenting what she digs out and repurposes.