[luau] "open source" books

[Charles Lockhart]

Maybe an odd topic question, but was wondering if anyone has or knows of 
statistics for the success of books that have free online versions 
available?  For example, the o'reilly Linux Device Drivers 2nd Ed. is 
made freely available (http://www.xml.com/ldd/chapter/book/index.html). 
 I'm trying to find out if this has had a positive, negative, or 
negligible on expected sales.

I keep trying to find this info, but I think I'm asking the question wrong.
Thanks,

-Charles