NP - Book sales info question for p-list

[Matthew Cissell]

Dear fellow P-listers,
 
    In Ian Rankin's book review of AD he writes: "The latest paperback of VL has gone into eighteen printings in six years, hinting at a readership beyond the groves of academe"
 I was wondering if anyone had advice on how to gain access to book sales information. I know that Publishers Weekly has the info but I seem to recall it not being free or cheap. Any ideas?
 
MC