Just so you know, there are many different types of sales. When I saw your header, I thought you meant "B2B (product) software sales." Many of the responses are about that.

But product companies are very different from agencies (i.e. consulting/services companies), which it sounds like you're trying to do.

A great place to start is Hubspot.com. Just do some of their courses. AND also try to talk to salespeople, and "agency people" in real life. It's a completely different mindset - not something you can learn intellectually; you have to let it soak in over time.

One great way is to try to sell anything in real life. You'll learn so much more from that than from a book.