Foliobook 4 solution
In Foliobook 4 you need to use a long press gesture (hold down a single finger for a couple of seconds) in order to enter edit mode.
If you still find you can't edit anything, holding down a long press doesn't enter edit mode, then you may have locked the app by mistake or just have forgotten how to unlock it.
Foliobook 3 solution
In Foliobook 3 you need to use a double tap gesture in order to enter edit mode.
If you find you can't edit anything, double tapping doesn't enter edit mode, then you may have locked the app by mistake or just have forgotten how to unlock it.
Additional Fix for all versions
To unlock the file, just leave Foliobook and go into the iPad's settings app
Scroll down the left hand side and find Foliobook
Then on the right hand side you can find the switch to unlock Foliobook
NOTE : In some versions of iOS there is a bug which causes apps not to appear after some iOS system upgrades. If Apps do not appear in Settings, you must power off and on the iPad. This should cause the app to reappear in settings.