FYI: An ebook framework is a collection of source code that you can use to make an ebook app. Think of it as all the parts of an app but the content (which you'll need to provide).
The basic version of PugPig is free, and the more advanced options are available under license. You have the option of paying for advanced interactivity, a live connection to Drupal/Wordpress backend (think magazine app), or even a complete end to end hosted publishing system.
I can't comment on the technical merits of Pugpig vs Baker, but I will say this. The demo apps that Kaldor currently have in iTunes generally respond a lot faster than any of the ebooks made with the Baker framework. I don't know why but I have noticed that the ebook apps made with the Baker framework are uniformly slow.
BTW, when you submit an app to iTunes there's always the chance that Apple might decide you belong in iBooks. That alone might be a good reason to go for Pugpig's more sophisticated options.