Browse to a book, and click the "Subscribe" button. Podiobooks will give you an RSS URL (looks like
http://www.podiobooks.com/.../book.xml). From the Podtrapper main screen, Menu->Add Podcasts->Enter urls manually. Type or copy the podiobook RSS url into the screen. You can enter in multiple urls at once, as long as they're separated by white space (space or enter after each URL).
I find the best way to enter URLs is to email them to my device, then copy/paste the whole block from the email to pt.
Note that you need to create a free account with podiobooks for this to work.
Also note that podiobooks.com will normally add episodes one at a time on a set schedule. You can change the schedule on the "Subscribe" screen, or there's a link to "release" all episodes at once so you can listen at your own pace.