1) You lent it to someone
2) Propping up a piece of furniture
3) You've moved house since owning it and its in the 'wrong' place
4) Its crept under your bed
5) Its in the bookcase all along
6) You left it in someone's car on the way back from a book signing
7) ITS BEEEEEEN STOOOOOLEEEN (gasp) by a very strong person.