I generally recommend the book Founding Sales (available for free online), but it's targeted at SaaS founders.

But you're actually doing something even more common: running a consulting business, and there's plenty of content on that for just that reason, so I would go find content on how to scale a consulting business, e.g. this seems like the start of a thread to pull on https://training.kalzumeus.com/newsletters/archive/consultin...