Yeah it looks nice but it´s also againt TOS since the term "Boeken" means Books and is positioned as such it is misleading the visitor in thinking the ads are actual website content. The blending has been done to well in this case.
I think if the owner would ever verify with Adsense support the message would be it is against TOS.
You can however do it like this,
and it would look just as good and unintrusive.
Granted the CTR would be less but the high CTR would be due to the fact it´s misleading.
Damion, very nice recommendations.. I have confirmed from few other webmasters too, using any heading other than Sponsored Links or Advertisement (or equivalent in other language) isn't allowed according to Google TOS.