I'm sorry but iframe widgets will have their code stripped out by software here to preserve security. Are you the author of the book? Are you reviewing books?
In a nutshell no blogger initiated advertising, retailing or reselling the work created or services provided by anyone other than yourself is allowed on free hosted wordpress.COM blogs. E-commerce transactions via shopping carts and the like cannot be conducted on free blogs from and being free hosted by wordpress.COM
The advertising exceptions noted in support documentation are for high traffic blogs that qualify for and are accepted into the Ad Control program, and for extremely high traffic blogs that qualify and are accepted into the paid VIP hosting program.
The exceptions noted in support documentation with regard to affiliate links are found here:
Affiliate marketing blogs: Blogs with the primary purpose of driving traffic to affiliate programs and get-rich-quick schemes (“Make six figures from home!!”, “20 easy steps to top profits!!”, etc). This includes multi-level marketing (MLM) blogs and pyramid schemes. To be clear, people writing their own original book, movie or game reviews and linking them to Amazon, or people linking to their own products on Etsy do NOT fall into this category. http://en.wordpress.com/types-of-blogs/
If you have any further questions about advertising and/or affiliate links on a free blog from and being free hosted by wordpress.COM please contact Staff directly using this link. http://en.support.wordpress.com/contact/
If you do require an ecommerce site, advertising and/or affiliate links on your blog that is not allowed on wordpress.COM free hosted blogs, you can hire a web host and get a free software install from http://wordpress.org Refer to > http://support.wordpress.com/com-vs-org/