Basic definition of theft
(1) A person steals if he dishonestly appropriates property belonging to another with the intention of permanently depriving the other of it.
(2) A person who steals is guilty of theft; and "thief" shall be construed accordingly.
Each of the points of proof must be satisfied in order to obtain a conviction for theft. The most difficult will be "dishonestly" because the logbook was arguably appropriated with your consent.
kaz3g,
The property was not appropriated - it was bailed into the flying school's custody for a specific reason.
It was never in the owner's contemplation that the book would become the property of the flying school, and I am sure there is evidence of payment by the owner to purchase the logbook.
For the flying school to attempt to retain posession of the logbook beyond the licence granted them by the logbook's owner is (if I remember correctly) "Conversion".