Reddit reviews Shipley Proposal Guide, 4th Ed.

A lot of professionals are fans of Tom Sant while others speak highly of the Shipley approach.

Personally I think the best advice is this: know your audience. The more you know, the more you can tailor your content so that it speaks to their goals, and addresses their pain points. So while I'm not client-facing, I rely on sales/account managers to tell me all they know about the prospect/client. At the start of every project, I'll ask them questions so I can get an idea of how we got to this point, and what their key motivations are.

As for moving over from non-profits, I think the skills are transferable, but you may have to target Bid Coordinator roles before you can make the jump to Bid Writer or Bid Manager roles. You may have more experience than me, in which case ignore my advice!