I thought the comment on exterior differential forms was interesting. I always wanted to delve into those and better understand what a dx all by itself was when separated from dy/dx by simple manipulation. Loved his comment "We justify this sudden introduction of differentials by saying that this is 'just another way or rewriting the differential equation,' or some equally atrocious lie."

I am now lusting after http://www.amazon.ca/Exterior-Analysis-Using-Applications-Di... but it's a bit pricy for a indulgence purchase!