For those who don't know, Paul Lamere, the author, is a really cool guy that works over at The Echo Nest (recently acquired by Spotify). I had the pleasure of meeting him and chatting with him a few times last summer while I was interning there and he's basically a music+tech genius. Lots of the echo nests coolest api demos are all his doing. I'd link to stuff but I'm on my phone, but definitely browse through his blog. If this is your area of interest, Paul is the guy to read.

edit: on my comp now, so I can add this link to some cool EN API demos, many of which are Paul's if I recall correctly: http://static.echonest.com/labs/demo.html