Chris from Catdynamics is currently running an extensive survey on the buying habits of people involved in web development, seo and online marketing.
It looks like the survey will provide some interesting data
I must admit I found myself answering "other" a fair amount, though I might not be typical.
5, 12 & 19 Other (interest plus to see if it would be of interest for an affiliate review / promotion)
22. For me personally it is either direct email from the publishers, or an affiliate recommendation either on the web or via email.
A second option, though still primarily by affiliates would be
