Brennon Dunn has a lot of great free blog content and an excellent podcast about all things freelancing / software consulting. Would highly recommend as his content has helped me a lot over the past year, mainly regarding the marketing/selling, billing and dealing with clients sides of things: https://doubleyourfreelancing.com
Brennan Dunn's stuff is great, and I know freelancers who have been very successful in part thanks to him. I'd recommend starting small if buying products though - he had a $50 book version of Double Your Freelancing that was worth it, but at one time he also had a $2000 online course version which probably isn't the right choice for everyone.
Freshbooks have a fantastic free PDF called Breaking The Time Barrier that is about charging for value, not for your time. It's worth reading even if you don't use Freshbooks (I use Harvest instead). The PDF was actually recommended to me by a client, who I think was giving me a hint that I was charging unsustainably low prices.
