First, remember that there’s a simple trade off here: the more you offer, the more likely it is that someone will sign up for the bonuses.
Now, that’s not just about value. Yes, if you offer three bonuses, they have more perceived value than if you just offered one.
More importantly, however, it gives you the opportunity to deliver your content in multiple formats. Different people learn—and like to consume information—in different ways. Some people want to watch videos, others want to read. Some people want to listen to audio, and some just want to read a few bullet points. Some people want a picture they can take in with a single glance on their screen, others want something they can hold in their hands.
If all you’ve got is a video, then a large proportion of your audience isn’t getting what they want, and they may decide they don’t want your bonus at all.
If you have a set of video chapter summaries, you can turn those into MP3s as well, and get a designer to create an infographic of your main diagnostic process (if you have one). Finally, create a checklist that the reader can print out and complete. Now you’ve addressed the needs and preferences of your entire audience, and everyone will find something valuable in that bonus stack that they want to get.
It’s not about creating new content. It’s about taking content that’s already in the book and presenting it in different formats for people to consume.