[abecedarius]

About Prolog-as-a-library, you might like to see how it was done for Lisp at http://norvig.com/paip/README.html

I guess for C the API would need to be very different like Lua's C API, but that's life.

[silentbicycle]

I've got that book, too. :) (And I recommend it to people all the time...)

I'm thinking something like Lua's C API, both because I know it well and because its design does a particularly good job of bridging C and a dynamically-typed language.