This website about thermal design of electronics is excellent:

Link: http://akemalhammar.fr/

Lots of really good calculators for things like heatsinks (both online and downloadable) and a load of other useful resources and info. All free!

I really recommend you check it out. It's been a big help to me.

Many thanks to the genius behind it, Dr Ake Mälhammar for kindly making it all available.