Twenty years is a very long time, and you can build up a lot of fears over those years. Try desensitizing yourself to having sex by imaging yourself doing it. Picture yourself and Mr. Widower doing the act. You'll get used to the idea that way. The book is illustrated and deals with many different sexual problems in plain English.
Sex is an experiential kind of activity, though, that's hard to learn from a book.
But you really don't have to be hemming and hawing and making excuses about having sex. He probably knows enough to take care of making it work. I suspect that you are somewhat worried about giving up your power and control, something that I've found is a real issue with women who've been successful in business. However, I'd suggest you let nature take its course and try sex again. Having sex is not like making a business deal.