Look for startups that are in the same vertical that your book is based on.

Reach out to the marketing leaders/team at those companies. They'll likely be very open to a mutually beneficial showcase - and even if they don't have podcasts, a Youtube video is a suitable replacement.[1]

I must ask - why only podcasts for promotion?

[1] Worked at three SF startups that did just this and were always actively in need of people/domain experts to bring on for interviews.