A super in-depth and fairly personal guide to everything to get your freelance operation going.

I am amazed at the amount of concreteness and obvious labour poured into this for free.
If you have any inclination of freelancing - setting your own costs and choosing your own clients - this takes you through the billing, tools, clients, rates, tricks, and more.

It is strongly focused on web development but some parts can probably be adapted to other areas.