Finding jobs in medical research that are attached to neither hospitals or universities is going to be difficult. Most career researchers will either teach (in a university) or under take professional practice (in a hospital for a medic).
You don't say if you have a PhD or not. Generally, if you want a job in medical research outside a hospital, you will ...
My take from your description is that the surgeon gave you his clear permission to publish without his name attached -- in the form of a written text message. Assuming that the wording in the text message is unambiguous, there ought to be nothing for you to worry about: He's expressed his opinion, and you're free to proceed.
Industry standard here is to offer royalties based off the revenue made. If Springer is selling your book, then each chapter download should not be free, and some percentage of that revenue goes to you.
I'd be surprised if anyone is willing to offer you remuneration based on downloads, since the same buyer might download the book several times.