If you want to become a highly successful speaker or trainer, do you need to have a constant stream of new content and new books?
The answer is that it depends. Certainly, there are many public speakers who are extremely successful and have put out hundreds of books and content. But what you need to do is question the industry that you are part of and your purpose. If you see yourself as primarily an author, the next JK Rowling, then go ahead and write a book every year or two.
However, if your focus is on some aspect of professional services, training, or negotiation, look at what your competitors are doing who are already very successful. For example, if I look into my field, the most successful, ridiculously wealthy, and wildly famous media trainer who is still in business, has never written a single book. Why? Because he is so busy with people paying him to train them for even one day and he is often booked 7 days a week. So, not having books has not exactly hampered his success.
I then look at my other niche- public speaking. One of the big fishes here is Dale Carnegie and he has put out a book- about a 100 years ago! While his advice in his book is good, it has still come far and little. I, myself, have written half a dozen books but also decided that writing books is not the primary way through which I want to market myself.
This is not supposed to be a knack on anyone, this is simply an analysis of my market space. Your marketplace is different- you need to know what your audience wants and how you can reach out in the most effective manner.