Yes, tried all that. In the end I downloaded another item to iBooks, forcing the app to go to the main menu therefore and letting me see my library again. The same problem occurred when I then tried to open the same PDF file so I decided it was possibly the file and, having downloaded YET another item to get to the main menu, I deleted the offending PDF. At the moment iBooks seems quite happy for me to open items and navigate around so I am hoping it was just a 'buggy' PDF!
Just out of interest, IS it possible to delete apps that Apple have loaded with iOS? None of the Apple-loaded apps get a cross on them when you set all the icons in "wiggle" mode to delete something!