"The Blueprint" by Brennan Dunn: A book/course about generating leads and getting clients.
"Selling the Invisible" by Harry Beckwith: A book about how to sell services (and how this is completely different from selling products).
"Secrets of Consulting" by Gerald Weinberg: Easy read, lots of war stories, actionable advice.
"Rework" by Jason Fried and David Heinemeier Hansson: How work should change when you do creative stuff.
"Praxisbuch für Freiberufler" by Svenja Hofert: A good introduction in freelancing.
"Steuern, aber lustig" by Andreas Görlich: Overview about bookkeeping and taxes. It covers only the legal situation in Germany.
"Freiberufler: Fit fürs Finanzamt: Buchführung, Rechnungen, Steuern & Co" by Constanze Elter: Deeper look on bookkeeping and taxes. It covers only the legal situation in Germany.
