In ZigBee stack, different layers communicate through SAP ( Service Access Point )
For example, APS communicates with NWK through NDLE-SAP.
However, are there any document, website or books explain these SAPS in detail ?
Such as are they bi-directional ? and some other knowledge in these SAPs
You can find the official ZigBee specs here: http://www.zigbee.org/zigbee-for-developers/network-specifications/zigbeepro/#learnmore
They are free to download after the registration.