I'm thinking about building a site on a subject that might have 20-30 different CB books on the subject. Is it okay to build one super site on a single domain with 20 different pages with reviews for each book and then link then all together or am I better to review one book per domain?
Well, it's up to you what you want to do. You can have your own blog with CB products reviews there, and use it as a landing page or you can get a domain for each product and build a landing page. I would go for the first option.
Here is an example: Site on Car Tires say Car-Tire-Information.com Here are the pages I would have: How to Clean Tires (Link to products to clean with) How to Fix Tires (Link to ebooks on how to fix) How to Rotate Tires (Link to tools needed to take tires off) All of them would be under one site but each one would be its own page.