Grayfox is right, but I think any bound book will do if the right information is put in it. They won't allow loose leaf binders because it would be easy to "lose" a page.
The term "bound book" is really a misnomer. The ATF allows a "series of loose leaf pages" to be used as long as they are numbered and contain the required information. I use a form prepared with an excel spread sheet. As a matter of fact, the "bound book" sold by Brownells consists of loose leaf pages.
I use a ledger book that has all of the pages glued together like a "real book". It looks very neat and is a very convenient size. Look on page #58 in the yellow Federal Firearms Regulations Reference Guide and it tells what info to put in each log entry.