[Koha] Koha API

We could use some help understanding how we go about querying the data in Koha to produce things like book cover images, new item lists, etc. Is there a good reference you can point me to for understanding how we can query koha from the website and display results? We (obviously) still intend to put the koha search itself in place, but I'm talking about programmatically getting at data. Any help would be appreciated.