Hi Dee and welcome to AW.
Hopefully someone else will have some advice on books you could read to help - there are certainly a lot of them out there, but I'm afraid I haven't read any of them. I am, however, a non-fiction editor (mostly educational books, but some more general non-fiction as well), so hopefully my advice will be of some use!
First of all, I would buy or borrow (you may well already own a few, in fact) other books in the area you intend to write. Look at the contents, how the information is presented/written and (very important) what these books are actually saying. What you need to do is to make sure that your own book will bring something new (for example, a new approach to diet/nutrition; a new regime which has proved successful with your clients; ????).
Once you are clear on what your 'something new' is, you then need to be certain that it really is new. For example, use Amazon and Google to find all the diet/nutrition books that are out there and read their blurbs and/or reviews to make sure that they do not offer the exact 'something new' that you have come up with. While you're looking at them, make note of who publishes the books.
Provided your 'something new' isn't already widely covered (if so, you'll need to rethink), go and look at the publishers' websites (i.e. pick a number of the publishers from the list you made when looking at the other diet/nutrition books. Many non-fiction publishers have a 'Write for us' (or similar) section on their websites, where they provide guidelines for potential authors. Make sure that you fit their criteria (e.g. qualifications and experience in your field) and find out what they are looking for. (Some publishers may state that they are not taking proposals on certain topics at the moment or may even have a list of topics for which they are actively looking for new books.)
Once you have settled upon your 'something new' and made sure that there is a market for it, you can then go deeper into what you are going to put in your book. A useful way to do this is to write a table of contents. To do that you need to decide how you want to structure your book. Ella's suggestion above about treating it like a consultation with a patient could be very helpful here. Think about the different stages you go through with a patient and whether that could be translated into the structure of the book. For example (bear in mind I have no real experience in this area, so this example might be completely wrong/useless!):
1 What does the patient (reader) want/need?
2 How does the patient (reader) currently eat?
3 Different regimes according to different needs/current practice
4 Steps to follow in changing attitude to food
5 Recipes (?) to help move through these steps
Once you have decided on the main structure of the book, you could then further split this down into the structure for individual chapters/sections. For example, you might want to include an introductory page or two for each chapter where you 'talk' to the reader as you do to a patient. You then might have checklists or tests that you use, which could be translated into a self-checklist/self-test for the reader. You might follow that with more detailed sections on specific topics (or foods - foods to avoid, foods that are good, etc?), with a conclusion to each chapter.
Once you have split down your contents and structure, you will then have small, manageable chunks to write (e.g. a couple of pages on each subsection). Use Ella's advice on how you talk to your patients and also use any documentation that you have created for yourself to help (e.g. checklists as mentioned above?).
Once you've written one chapter (or even one section) get some people to read it (you could post it in the Share Your Work board here, but also try to get a number of other people to take a look at it, too). Don't necessarily take everything these readers say as gospel, however, hopefully their comments will give you a general idea of particular areas in which you need to change (e.g. are you being to personal, too patronising, too unclear, etc.?). Look over your chapter with their comments in mind and make changes that you feel will improve it. Then write another chapter keeping these changes in mind.
Once you have three chapters (or three sections - depending on the length) written, ask your readers to look at it again. If you can find any other readers, that might be useful. (What about asking one or two of your patients to look at it? Or a colleague in the same field?)
Once you are happy with your three chapters/sections and have your contents sorted out (you might have decided to change your contents slightly in light of reader feedback or thoughts that occurred while writing the first chapters/sections - that's OK), you should be ready to put together your proposal. You will already have most of the information you need from the tasks above (i.e. the 'something new' that your book provides; how it fits into the market; details about what your book will cover, your own experience and qualifications).
Use the book you bought on writing proposals, but also make sure that you follow the publishers' guidelines (check whether or not they expect an exclusive view of the proposal, or whether you can send it out to multiple publishers at once). Send out your proposals and then forget about it. Generally speaking, it's probably better not to write the whole thing (in non-fiction) before sending out the proposal. If you get an offer there are likely to be changes the publisher will ask you to make (e.g. they might want to change the structure somewhat, or they might have requests for a change in writing style; they might also have a house style that you need to follow) and it will be easier to write the rest of the book with these in mind, than to rewrite it after the fact. [Note: it is possible that some of the publishers might expect you to have the book ready and finished; always check the publishers' guidelines.]
Note: The above is the way many authors work in educatinal publishing (apart from the proposal part, as books are usually commissioned in eduation publishing). It works particularly well for ensuring that all parts of a syllabus are covered and that the book is structured consistently throughout. It may not be the way you find it easiest to write. Try it out and if it works for you, use it. If it doesn't, try out other suggestions you get (from people here, from books, from courses, etc.) until you find the way that does work for you.
Hope that's of some help. Good luck with your book!