Track referrers and link them to actual purchases I am the owner of some content being sold through ibooks (http://www.sinkronigo.com). Some other websites are adding ibooks links to my books and I would like to know how many clicks to these links resulted in actual purchases. 

I have been reading about the Itunes affiliate program. As far as I understand this is something that the referrer should join and then I would not have much control over it.
The iTunes Affiliate Program will get you a bit closer to understanding the marketing of your book but unfortunately it's not a silver bullet.  

1.) To see the clicks that are generated on links to your book those other properties will need to use YOUR affiliate links.  Anyone using their affiliate links or a raw iTunes / iBooks link won't provide you with any insight.  

2.) You won't be able to attribute which clicks / referrers actually resulted in sales of your specific books.  iTunes reports back affiliate sales at a category level, not a product level.  

3.) iTunes Affiliate links are only active for 24 hours (or until the user clicks another affiliate links).  Any purchases outside that window are lost.

There are, however, some services, like GeoRiot (Disclosure: I am a co-founder), who offer some attribution reporting for iTunes products.  Here is a blog post about a project we did to solve exactly the problem you are wondering about - Introducing the Relative Conversion Score for iTunes