You can see the three PDFs (Olympus camera manual, HP printer manual, and AppleScript Language Guide) on the top shelf of the iBooks bookcase. Watch for another blog entry here with all kinds of iBooks tips.
UPDATE: if your iTunes is set to “Sync Books” you should drag your PDFs to the LIBRARY in iTunes, at the top left of the iTunes window. If you try to drag to the iPad itself it won’t work. You will also have to click the “Sync” button in iTunes when you are done dragging. If you’re synching “All Books” that’s all you do. If you are synching “Selected Books” you will have to check the box next to these newly-dragged books. Either way, it’s not hard.