I upgraded my early stock iPad 16 Gb to iOS5 and after that the app iBooks crashes all the time.
The only thing I use it for is viewing maps with bus lines I made in Illustrator and then saved as PDF-files. The PDF:s were then imported to iTunes library.
The maps worked well in iBooks before the upgrade. Now the app will launch, show my book list for a few seconds, then die.
I found a good replacement in the app Bookman Lite, but I would of course try to fix the iBooks problem too.
Available free space on the iPad right now is 3.2 Gb.
Happy new year!