Costs of borrowing a book in various London boroughs:

Camden £11.50
Greenwich £7.14
Hackney £10.07
Hammersmith £6.63
Islington £10.46
Kensington £8.54
Lambeth £10.29
Lewisham £5.77
Southwark £6.89
Tower Hamlets £9.90
Wandsworth £3.64
Westminster £5.91

The figures are from the Good Library Blog, and are calculated by dividing the library cost by the number of books loaned. It would be interesting to get the numbers of books loaned for each borough, which would let us derive an estimate of the total cash value of waste. It looks as though the figure for Camden could be over £7 per book loaned.  

Which suggests that taxpayers in Camden would be better off giving people book tokens and sending them off to Waterstones.