I think the book file info is stored on the Kobo website. There is a notice when you remove a book from your playbook, the purchase information remains on the website incase you want to reinstall.
Kobo books a proprietary and hidden. Though files are stored on the playbook so you can read off line you wont be able to find thm.
If you want to read on your PC download kobo book reader for PC. Connect to internet with PC or PB and your book marks will be synchronized.
You can also download many books in Adobe eBook format.
You can also strip the books of DRM with software from the net if you prefer being able to read and access across platforms. Search Kobo and DRM. You will likely find a post I left on the subject.