http://www.ibm.com/developerworks/lotus/downloads/toolkits.html#notesdomino is where you will find version 8 of the C api which is described as “A toolkit that lets you control Domino components using Domino’s C API”.
It been so long since the last version was released that I’ve almost forgotten the last time I used it to extend an application. Time to dust off my notes and kick the tyres on this new release and make it worth the developers while to have updated it.