Earlier this fall, one of the most important business books of 2017 was released -- the new work of Dorie Clark, a professor at the Duke University Fuqua School of Business.

The book is called Entrepreneurial You, and it's the third in a series that includes Reinventing You and Stand Out, which help people transition their careers and find their breakout idea. 

However, her latest book takes a brutal look at the state of today's gig economy, and forces us to ask how we can possibly rely on a future with a single income.

Having a single source of income is a thing of the past.

As Jenny Blake put it in her own book, Pivot, the world as we know it is in constant flux, largely due to technology, the evolution-minded Millennial generation, and the untrustworthy environment created by corporations. 

Therefore, "with the changing economy," Clark recently told me, "the best career insurance is having multiple streams of income." 

But here's the essential part: Diversifying income streams isn't just for entrepreneurs like both Clark and yours truly. Instead, both of us agree that -- as Clark put it -- "developing a side income stream is a great way to earn money on the side, mitigate risk and learn valuable new skills," even if you love your day job and can't see yourself ever leaving.

Unfortunately, as many people found out in the financial crisis, loving your job and never wanting to leave don't make for a stable base of future income. And largely thanks to the crisis, Clark is just one of many entrepreneurs that is calling for people to diversify their income streams and hedge against the risks of your day job.

In fact, Harvard MBA and inventor of FOMO, Patrick J. McGinnis, has made a career out of encouraging people to become a "10% Entrepreneur," and use a side hustle as an income stream, creative outlet and insurance policy against our rapidly-changing world.

The good news is that the options for a side hustle are endless.

With so many opportunities out there to build a personal brand and develop a side gig, the process for doing so can seem overwhelming.

That's why I consider Entrepreneurial You to be so powerful: It shows us not only why it's essential to develop a side hustle, but also how it's easier than ever to put yourself out there and do it.

Then, Clark shows how real people are taking actionable steps every day to diversify their income streams and develop new sources of revenue. From podcasts to speeches and from consulting to infoproducts, she gives you a wide variety of avenues to choose -- and action steps to get there.

I've long been a believer in diversifying income streams, which is partly why I recently launched a new coaching business to help Millennial entrepreneurs reach $1 million in revenue. But even I didn't realize all the ways I could market my new business and make it successful until I read about the stories of today's best internet entrepreneurs in Clark's book.

The best time to start is now!

Yesterday I interviewed Carrie Kerpen about her new book coming out in January. She's built a podcast called All The Social Ladies, and that's given her a central hub around which to build her platform. 

So I asked Kerpen, "if you were starting today given the saturation of podcasts, would you start a podcast?"

She admitted that of course it's harder now than five years ago, but said she would absolutely start a podcast today. Why? Because niche markets are always looking for good, accurate, informational and inspirational content.

That means it's not too late, which is great news for all of us.

When you develop your niche target and create an offering to match, you'll need to build an audience somehow. In order to build your personal brand and develop your side hustle, you'll need to put yourself out there one way or another -- blogging, podcasting, releasing videos or hosting a webinar.

All of those avenues are available to you and can be learned. Remember: no one else is slowing down to wait for you, which means the time to start your side hustle is right now.

The best news is that you don't have to go it alone. tap into the expertise of Clark and her interview subjects, and you'll have a huge advantage over the competition. 

Then, make sure you do what most never do: take the first step forward today.