apple make you wait a year to read your books if you forget to logout of ibooks itunes istore on any computers you have used
You Have Already Authorised Five Computers
You can still make purchases on this computer, but you will need to transfer them to an authorised machine or device to play them. If you wish to play purchases on this computer, you will need to first deauthorise a currently authorised computer.
BTW You can deauthorise all your PCs in 12 months time