Do your research first. All nonfiction books require some research. Since this is the most time-consuming aspect of writing the book, have your research finished and sitting by your side as you write. This will allow you to focus strictly on the writing aspect. If you research as you write, you run the risk of getting off the writing path and getting too involved in the research.
Determine approximately how many pages your book will be. Nonfiction books can range from 300 pages to nearly 1,000, depending upon the scope of the subject. Aim for a goal of 10 pages a day. This will give you a 600-page nonfiction book in 60 days. If 10 pages is more than you can do daily, adjust the page number of your book and daily writing quota to reflect what you can realistically accomplish.
Prepare the research you'll need for the next day's work. Research should be ready to go and organized in the order you need it. This will help speed the process of actually writing the book.
Read your previous day's work on the following day. Ideally, you would want to do a second draft after you've fully completed the first draft, but to get a your book finished in the allotted 60 days, you will need to edit as you go. If you are meticulous, you should have a fairly clean manuscript at the end of 60 days.
Set your book aside for a week or two before you do your final read through. Although you've edited as you've written, there's a good chance the book will need quite a bit of polishing. Now that you've written the book in 60 days, take the time to line edit for not only spelling and grammatical errors, but for poor sentence structure and clarity.