Hi T., that’s spam. Whether inbox spam, contact form spam, or blog comment spam, spam is spam. It’s frowned upon. Professionals don’t recommend that tactic. I don’t endorse that.
No one appreciates receiving junk email in their inbox or via their website contact form. No one.
If you want to promote your book, I really think you should go the route I suggested years ago with a proper PR person. They get you legit interviews (written, audio, video) with credible experts that have audiences that would be more likely interested in your message rather than spamming random website contact forms. I’ve since learned there are PR professionals that use a pay for performance model so there’s less risk.
All that content — articles, podcast audio and video — will continue to live on at those targeted business sites so new people will get exposed indefinitely. Unlike a “spray and pray” email spam or contact form spam that may harm your reputation.
Another alternative is sponsor emails. It’s a form of targeted advertising. (example: A brand advertises their color contact lens free trial to teen girls by letting Seventeen magazine send an email to their 70,000 email list of opt-in teenagers.) This isn’t spam because the email is sent by the publisher to their existing opt-in email list, as opposed to you sending to strangers that don’t know you and didn’t request to receive anything.
I hope this is helpful.